My Own Grampa

"''My little baby then became a brother-in-law to dad

And so became my uncle though it made me very sad

For if he was my uncle then that also made him brother

Of the Wider's grown up daughter who of course was my step-mother''"

- Lonzo and Oscar, "I'm My Own Grandpa"

Simply put, a Stable Time Loop where one is his own ancestor. Of course, this implies an Identical Grandson, and an Ontological Paradox which results in a quarter of one's DNA being created from out of nowhere.

It should be noted that the song which named this trope, I'm My Own Grandpa, is not an example of the trope, as it includes no actual time travel. It describes the singer's rather complicated family life.

If they're just pretending to be their grandchild, then that's My Grandson, Myself. See also Kid From the Future, for more time-travel and offspring related hijinks, and Time Travel Romance, which could lead to this trope.

See also Grandfather Paradox.

Anime / Manga

 * Pierrot Bolneze in Yakitate!! Japan was abandoned at a circus as an infant, but by Kazuma Azuma's history-changing Taima Ja-pan, he goes back to his childhood and ends up saving his mother from Death by Childbirth. When Pierrot comes back to current time, both his parents are alive in this time line. In this version of reality, he became a clown and was named Pierrot because his parents were so impressed with their old butler, the adult Pierrot from the future who was abandoned and raised in a circus.
 * Tsubasa Reservoir Chronicle - oh my, oh my, OH MY !! Following Cloney and C!Sakura's Heroic Sacrifice, they re-incarnate in the past, meet, marry and then comes the WHAM moment of the story, when they become the parents of R!Syaoran, as the reader drops brain dead at this point. Then it only gets worse as more logic is thrown of the window...
 * Also creates more biological screw when you realize that Father, Son and Grandson all have the same DNA.
 * This all makes a weird sort of sense when you take into account that the Big Bad of the story has been making an attempt to rewrite the basic laws of reality, screwing up everything and causing things that should not make sense, or are flat out impossible to happen.

Comic Books

 * Played with, but averted, with the Marvel Comics villains Kang the Conqueror and Doctor Doom. Kang, the Ur Example of the Conqueror From the Future, was for some time believed to be a future version of Doom, and the two worked together (and double-crossed each other) on many occasions, culminating in the Secret Wars maxiseries, in which Doom had Kang disintegrated by Ultron (he got better with the help of a Reality Warper). Later, however, it was revealed that Kang wasn't really Doom's future incarnation. He was actually either Mister Fantastic's father or distant cousin or some such thing (it's confusing).
 * What complicates it is that Reed Richards' father and Kang are both named Nathaniel Richards, but are completely separate people. We think.
 * Kang is a bit of a mess. There is another character, more or less a good guy, who's a future version of Kang, and there turns out to be a rather large group of Kangs, from different points of their own time line or from parralel realities, that cooperate and occasionally congregate for various purposes, then there's a larger group of Kang-analogs that are clearly Kang but have different faces, different genders, different species...
 * It is supposed to be impossible in the Marvel universe to become our own ancestor because you can't actually travel into your own past. If you try, you end up in what will already become a different universe than the one you left (for example, Rachel Summers tried to avert the future she came from, only to eventually find out she had arrived in a different time line).
 * Alan Moore's "Chronocops" strip for 2000 AD sees the time cop narrator arresting his partner, who attempted to marry his own grandmother as a young woman after he suffered a mental breakdown. The narrator himself ends up retiring from the Time Police after this and marries his partner's grandmother himself. His (now former) partner says that he's going to kill him when he gets out of jail.

Fan Fiction

 * Referenced in Kyon: Big Damn Hero, with Kyon recalling someone saying he looked identical to his grandfather at that age. Given how much time-traveling he does...

Film

 * The title villain of The Terminator is the most famous western case, as its travels back in time resulted in it "fathering" its own master Skynet.
 * Time Rider, where the hero went back in time and became his own great-grandfather.
 * Marty McFly spends most of Back to The Future avoiding becoming his own father. He succeeds, but instead becomes his own namesake.

Literature

 * Chuck Palahniuk's novel Rant - many of the major characters  with the theory that
 * Not only that,   Lots of   needed for that.
 * Played to the extreme in the Robert A. Heinlein story All You Zombies,
 * "By His Bootstraps" is an example by the same author of the "milder" version described above.
 * Time Enough for Love has a blurb on the back cover that claims this trope occurs, but it couldn't have happened in the novel, nor could it have happened off-stage, since Lazarus's arrival date in the past was after his own birth, quite intentionally (and Maureen is already pregnant with a younger sibling of Lazarus when he arrives). The blurb-writer got a few other details of the book wrong, and probably was working from a grossly simplified synopsis.
 * In ~The Hitchhiker's Guide To The Galaxy~, Zaphod Beeblebrox's father is Zaphod Beeblebrox II, his grandfather is Zaphod Beeblebrox III, etc. When asked why, he says it was an accident involving a time machine and a prophylactic. His great-grandfather takes to referring to him as "Zaphod Beeblebrox the Nothingth" as a insult because of it.
 * Though The Hitchhiker's Guide does state that becoming one's own grandparent through time travel is nothing that a significantly well-adjusted family can't deal with.
 * The Man Who Folded Himself,
 * Except, of course, for the lawyer, Mr. Briggs (or something).
 * In a book called "The Mirror", a young woman switches bodies/eras with her grandmother, and then lives her life. (in other words, she is the one who has the great romance with the grandfather, who is thought by the family to be psychic, who gives birth to her mother...). At the beginning /end of the book, she dies as her grandmother, while her grandmother then lives the rest of her life.
 * Spider Robinson's "Have You Heard The One..." features a visit to Callahans Crosstime Saloon by a time-traveling salesman, whose plans are foiled by
 * The first edition of Harry Potter and the Chamber of Secrets has Dumbledore stating that Voldemort is the "last living ancestor" of Salazar Slytherin. Among those who realized what that actually means (in context, it's easy to miss), there was some wild speculation, especially since . Of course, it was simply a mix up with "descendant". Some people use "ancestor" in a sense that includes "descendant", but it's best not to for clarity's sake.
 * Robert Silverberg's time-travel novel Up the Line has a character who is trying to sleep with all his female ancestors, except his mother (as he puts it, "I draw the line at abominations"). He's careful not to father any children, he is just seeking to cuckold all of his male ancestors (except his father because sleeping with his own mom would be sick).
 * Robert Silverberg's time-travel novel Up the Line has a character who is trying to sleep with all his female ancestors, except his mother (as he puts it, "I draw the line at abominations"). He's careful not to father any children, he is just seeking to cuckold all of his male ancestors (except his father because sleeping with his own mom would be sick).

Live Action TV

 * Charmed: Piper Halliwell's past life, P. Baxter, was the great-grandmother of her present life. Points for being biologically possible, as the soul is the same, not the body.
 * In an episode of Star Trek: Deep Space Nine, Bashir wonders about this when he meets a woman on Kirk's Enterprise whose last name was that of an ancestor's, but it proved to not be true.
 * Red Dwarf's Lister is his own father. Long story.

Video Games

 * Achron: The alien Grekim are able to pull this off in gameplay (see here for directions how). It has little practical use though, so it doesn't happen often.
 * Digimon Tamers Brave Tamer establishes that a combination of this and Timey-Wimey Ball to Mind Screw levels resulted in the birth of
 * Add to that that Chimairamon was created by  as a direct result of him having been hit by Milleniummon's   years earlier. The irony being that   ended up setting the events in motion that screwed up Ryo's life by Taking the Bullet for him.
 * Oracle of Tao has a version of this that doesn't involve time travel, but rather a God in Human Form situation, where God impregnates her mother, even though God turns out to be her true form. There are so many things wrong with this.

Webcomics

 * Irregular Webcomic: Adam and Jamie form Myth Busters attempt to do this. They screw it up, becoming each other's grandfather, and therefore their own great-great-grandfathers.
 * Wicked Powered: Taken to its natural extreme. The main character was sent back in time twice (once as a woman and once as a man). He arrived in the same time on both occasions. The two instances met, fell in love, got married, and gave birth to a child. Take a guess as to who this child was.  And apparently, being your own parents makes you immortal. Or something.
 * Homestuck has the most bizarre form yet.
 * It gets worse:
 * It gets much worse.
 * It Got Worse... or better, maybe?
 * This also happened with

Western Animation

 * A well-known example is the Futurama episode, "Roswell That Ends Well", where Fry unwittingly becomes his own grandfather. Surprisingly, this actually became a major plot point in some later episodes.
 * To clarify, Fry meets his grandfather, Enos, in 1947 Roswell and becomes so afraid of getting him killed (and thus not existing) that he winds up causing him to die. Later, Fry (still perplexed that he is alive) comforts the woman he believes to be his grandmother. She drunkenly propositions him, and Fry deduces that since he's alive, she must not really be his grandma. Boy was he wrong giving us the terms "nasty-in-the-past-y" and "past-nastification".
 * By the way, this trope is why Fry's the only one capable of saving the universe and also explains why Fry not going into the future in "Anthology of Interest" causes the universe to self-destruct. By not getting frozen, he couldn't end up becoming his own grandfather, and thus caused a major paradox.
 * In an episode of Xiaolin Showdown, Dojo warns Omi that by using the Sands of Time, "You can end up becoming your own grandpa."
 * Averted on Count Duckula, in which the character only claims he's his own father, grandfather, etc. He really means that each time he comes back from the dead, he remembers nothing and is effectively reborn with a fresh personality.
 * Fairly Oddparents has an example. In the Crash Nebula episode, Wanda says that there is an episode where Crash discovers he's his own great grandfather after traveling through time.
 * Family Guy has Stewie taking the place of his ancestor Leonardo da Vinci when he is killed by Bertram and managing to pass his own genes down before freezing himself and taking The Slow Path back