Mono-Gender Monsters

Along with Monster of the Aesop there's a tendency for all the monsters in a given show to be men or women, rarely will there be a mixed bag. The exception is if the The Dragon or the Big Bad are a different gender and deign to fight the heroes. And much like Monster Misogyny applied in reverse, guy heroes will only fight guy monsters, and girl heroines will only fight girl monsters. Though girl heroes fighting male monsters is not uncommon either, male is kinda of the default gender for Mooks(if not the default gender in fiction).

This also tends to follow the predominant gender among the cast. Mostly girl heroines? Then it's female monsters. Mostly guy heroes? Then it's male monsters.

There's really two reasons this happens:
 * 1) It's a lot easier on the costume/animation department if everything is fairly uniform across the board for the generic cannon fodder and slightly less generic Monster of the Week. There's also an easy story justification when monsters are involved: simply put, they are all modified forms of one design, pressed from a single mold, created by the same spell or, in the case of the first generation of Imperial Storm Troopers, cloned from the same guy.
 * 2) In the case of where the single gender protagonist(s) fight single gender monsters, this is probably to avoid Unfortunate Implications. Violence of men against men and women against women isn't viewed with the same sort of sensitivity as men against women. By reserving the men against women for developed characters (where you can develop very good reasons why they need to fight each other) and not having it for the Exclusively Evil Mooks that get steamrolled every episode, you can avoid accusations of the piece being inherently misogynistic or misandristic.

See also One-Gender Race, Designated Girl Fight. Contrast Female Monster Surprise.

Anime and Manga

 * In Guyver, no female zoanoids are ever featured (though a couple appeared in the American live-action movies).

Film

 * The Lord of the Rings movies had only guy orcs. Possibly justified since they're asexually reproducing (though Word of God would disagree, maybe; Tolkien wasn't known for being specific about the sexual predilections of orcs)...yech.
 * This is true for the movie version; however, Tolkien had written that "Orcs multiply after the fashion of the Children of Illuvatar", i.e. elves and men. (Written here because there's no Literature section on this page.)
 * In just about any fantasy setting, it is rare for female goblins, orcs, trolls, ogres, or giants to be mentioned. There are of course exceptions (such as Xanth, Shrek, and tabletop games run by particularly forward-minded Game Masters), but they tend to be rare.
 * Though, in the final movie, there are two orcs that appear to be strikingly female, like the one who orders for the catapults during the Minas Tirith battle.
 * Some orcs (such as the one who inspects a freshly born Uruk-Hai in Fellowship) were played by women, but whether they were meant to be orc women, who knows...
 * Star Wars Episode II and onward had the Storm Troopers, clones of Jango Fett.

Live Action TV

 * Power Rangers tends to limit itself to two female monsters per season, though there is always at least one female Dragon.

Tabletop RPG

 * Dungeons and Dragons has a few female orcs in the fiction (there is one particular in the latest few Drizzt books). There are rules for them, in that orcs and many other humanoid monsters have tribal rules in their monster entries, covering how many females/elders/infants/so forth should be part of any group of Monster X. Somewhere between the inattention to detail of most games, the vague distaste for genocide, and the fact they don't provide much in the way of XP, they tend to get ignored.
 * For orcs, it's justified that you don't often fight females—the males usually treat them as breeding stock whose place is in the home or otherwise away from front lines. So in any scenario where conflict with female orcs is possible there's also probably orc kids and babies around, and any sane DM would probably advise players of non-evil characters to tread very lightly so as not to upset Mama Bear.
 * The Forgotten Realms novel Evermeet quotes an Orc proverb "If Gruumsh had intended females to lead, he would have given them bigger muscles."
 * And for trolls it's averted, since they're matriarchal.
 * This is typically applies to gnolls too, which is a little strange since real life hyenas are matriarchal.

Video Games

 * Donkey Kong Country: ever see a female Kremling before Barrel Blast?
 * The Super Mutants of Fallout all look male, but it's revealed in files that the FEV virus which creates them makes both genders visually identical (secondary sexual characteristics like breasts and high-pitched voices disappear). Indeed, in Fallout: New Vegas, there are several female Super Mutants who, yes, sound exactly like the men (though they don't necessarily act like the men; one is a sweet old granny).
 * All of the Covenant Aliens in Halo are male.
 * Samus Is a Girl, yet all space pirates in Metroid are referred to as male. In fact, pretty much everything Samus fights is either male (Ridley, Space Pirates, Kraid...) or gender-neutral (most of the wildlife, the titular creatures, robots...). Even Dark Samus and the SA-X technically have no genders. As for the Mother Brain, "she" seems to refer to "herself" as female in the manga (and in Captain N), but "she's" a giant brain in a jar, so "she" may not count. The only enemy Samus ever fought who was definitely female was.
 * The Orks of Warhammer 40000 are basically "drunk football lout" crossed over with "genocidal zealot", and are universally ridiculously masculine. Justified because they reproduce asexually (biologically, they're a lot like fungi).
 * This has lead to a lot of interesting little bits of in-universe lore, though. Such as if a planet's ever invaded by Orks, even an Exterminatus might not be enough to get rid of them, and their spores, completely, and their version of a doctor being able to work in a perfectly effective manner, despite being a psychotic chainsaw nut that harvests spare parts and stick them onto those in need at near random.
 * The spores just don't make more orks; they also make the fungi and squigs they eat, and the lesser greenskins to farm and help the orks. In other words a single ork can make an entire ecosystem. (This is the main reason why orks are so good at fighting the Tyranids.)
 * The orc part is averted in the Warcraft series, a female orc was seen as early as Warcraft I. Though being an army building game, its mostly played straight with the other races, female humans alone weren't seen until the third game and it took till World of Warcraft for females of most of the other races such as Trolls, Tauren, and Dwarves to show up on screen.
 * Pretty much played straight with most other monstrous races. Ever seen a female ogre, sea giant or fel orc?
 * The later Wizardry games. In Bane of the Cosmic Forge, all dwarves, giants, gremlins, knights, samurai, ninjas, and rogues are male. In Crusaders of the Dark Savant, all Rattkin, Gorn, Munk, Dane, and Umpani are male. There is only one T'Rang female, justified in that she's a bloated egg-laying queen, and Wizardry 8 adds the Rattkin females... most players rather wish they hadn't.
 * Pokémon has quite a few Always Male monsters: The Tyrogue family, Tauros, Gallade, Volbeat, Mothim, and the Rufflet line.

Anime and Manga

 * Cardcaptor Sakura mostly fights cards that seem female when they aren't too ambiguous, but this might be just because they're so cute looking.
 * With some of the things implied about their in-universe creator, it'd be unsurprising if he specifically decided he wanted them cute looking.
 * Cutie Honey may possibly be the Ur Example, being the forerunner to the Magical Girl Warrior subgenre of Magical Girl series. True to the author's style, the monsters, while female, were always hideous.
 * In what is probably a nod to Sailor Moon, the Pretty Sammy series employs exclusively female "Love-love Monsters", which are mundane objects animated into anthropomorphic creatures with similar traits.
 * Based on their voices and booklet art, all of the Marriages of Magical Girl Lyrical Nanoha StrikerS Sound Stage X appear to be female.
 * All of the Dolems in RahXephon had female characteristics, their avatars were women in skimpy outfits, and their Mulian controllers were all women as well. There are two exceptions, both of whom are plot relevant to the lead and the tormented leader of Terra.
 * Sailor Moon had female Monsters Of The Aesop, and only bad guy lieutenants and Monsters of the Week made from the Victim of the Week were guys, and obviously only if that victim was a man. One exception is in the SuperS season where some of the lemures are guys.
 * Since this is mainly a trait of the TV show, this is cited as one of the homages to the above-mentioned Cutie Honey.
 * Zig-zagged in Claymore. Most of the monsters the characters face are Male Youma, but 99% of Awakened Being are female, prompting a shocked response from one character on there even being a male Awakened.
 * As could be assumed from their name, the witches from Puella Magi Madoka Magica are all female... not that you could tell by looking at them.

Video Games

 * The Helazoid race in Wizardry VII: Crusaders of the Dark Savant are a thriving all-female culture. Justified by their use of advanced technology to reproduce and only produce female children.
 * In Scarab of Ra, the pyramid is infested with monkeys, cobras, and lionesses.

Web Comics

 * The three peanut butter monsters in The Inexplicable Adventures of Bob are all female, since they're all genetically identical.