Mythology Upgrade

Need a totally awesome magical beast to make your show that much more awesome? Why make one up when mythology's done it for you? The Behemoth, the Leviathan, maybe a dragon or two. In works involving the afterlife, you can even expect Cerberus to make an appearance.

But wait...in their original forms, these guys weren't quite awesome enough! We need to give him NEW powers so he presents a real threat to our heroes (or our villains). Behold the power of creative license! Time for a Mythology Upgrade!

Cerberus not cool enough? Let's give him a human form! Raging dragons not doing it for you? Let's make them superintelligent and magic-resistant! Leviathan not dangerous enough in the water? Let's make him FLY!

Note that the Mythology Upgrade refers specifically to an already established creature of legend who gets entirely new powers, not one that just mysteriously got more powerful. Compare Public Domain Artifact.

Multimedia

 * Arachne is simply a progenitor of all spider in Classical Mythology. If there are monsters with that name in modern work, they are alway half-spider half-woman being with human-size, if not larger.
 * Cerberus deserves a special mention in any appearance he makes, since the original creature, depending on which myth you're going by, had snakes growing from his back and was formed by/had skin made of the squirming souls of the dead. Which practically makes many of his appearances a Mythology Downgrade.
 * It should also be noted that Cerberus originally only had one head, not the three he is famous for having. That didn't come until the Romans got to him (making this an Older Than They Think trope in and of itself). He was also often portrayed as an average sized dog, rather than the gigantic monstrous beast many relate him to. This is of course assuming that Hades is an average sized man, which is difficult to tell with gods...
 * Dragons are perhaps the greatest recipients of this trope. Look up old paintings of dragons and they're often smaller than the hero's horse. Besides their fire breath and animal ferocity, most myths didn't give them many additional powers. Modern sources, however, almost always depict dragons as gigantic beings of incredible physical and magical strength. Dungeons and Dragons is one of the worst offenders. See Our Dragons Are Different for the various permutations.
 * Earlier than Dungeons and Dragons, JRR Tolkien's dragons were huge, highly intelligent, possessed magical powers (particularly mind-bending magic), and were quite possibly incarnate minor deities, if evil ones. Word of God admitted a certain fondness for this particular creature, so he used them sparingly but when he wanted to go for grand effect.
 * Dragons were always described as huge; those paintings that show them as smaller than the knights slaying them are using symbolism to show that the heroic and chivalric knight is ultimately more powerful than the monster.
 * And the magic powers and wisdom come from the Asian dragons that are essentially minor gods or elemental powers.

Anime and Manga

 * An arc of the Yu-Gi-Oh! anime gave the Leviathan strange powers related to souls and children's card games. It could apparently make people immortal and/or "awaken the darkness in their hearts", and was said to be responsible for the fall of Atlantis.

Tabletop Games

 * Scion upgrade various mythology beings with modern elements. Example including Centaur as half-human and half-Harley-Davidson motorcycle, Scylla has machine replace its monster heads and Surtr's main fortress in Muspelheim can transform into Humongous Mecha.
 * White Wolf also generally did this with their Werewolf game lines. It's not enough that they're shapeshifting killing machines—to fit roughly in with the themes of works such as The Howling and Wolfen, werewolves also have ties to nature that give them access to the Spirit World and a special relationship with its denizens. Whether this relationship is "stalwart defenders" or "border police" depends on the gameline.

Video Games

 * Devil May Cry 3 had the legendary Leviathan as an enormous flying creature with a demonic immune system, which Dante has to fight through to kill the creature from the inside out.
 * Not to mention giving Cerberus some wicked ice powers. Or Beowulf turning from a human Badass Normal to a light-flinging demon. Or Geryon having time-control powers. Although, Cerberus's ice makeover does work nicely as a homage to the coldness of hell in Dante's Inferno. Complete with him guarding the gateway to hell (temen ni gru).
 * Final Fantasy pretty much owns this trope, since several of its enemies and almost all of its summons are taken from mythology, and then powered them Up to Eleven. List examples below:
 * Bahamut, who makes an appearance in almost every game, is the King and/or God of all dragons. In his FFX incarnation, he has a giant ring on his back which charges with energy to unleash his Overdrive, which is basically a giant blue laser.
 * Shiva, who goes from six-armed hermaphroditic deity to surprisingly well-endowed and definitely female ice goddess.
 * That may be a coincidence, as her name is evidently supposed to be Shiver but given Japanese language barriers, is rendered as Shiva.
 * Leviathan in Final Fantasy VIII indeed does fly.
 * Adrammelech, a pagan sun god who for some reason has electric powers and looks like a dragon.
 * It looks more like a flying goat
 * Castlevania: Circle of the Moon made it a giant, eyeball controlling, ooze gagging goat thing that pissed off players.
 * Cu Chulainn, Ixion, and many others bear almost no resemblance to their mythological counterparts.
 * Both Bahamut and Shiva's cases are really examples for Mythology Downgrade since both are way more powerful in the original myths. Bahamut is a whale-like Eldritch Abomination that is larger than the world (and the world is so small compared to Bahamut, since it only floating in a small crack on a bull's head, said bull is standing on Bahamut's back). While Shiva can burn the world simply by opening his third eye.
 * And while we're at it, said bull shows up in Final Fantasy VII as a tri-elemental summon. Still a bit of a downgrade, but hey, random elemental powers!
 * This isn't a Nethack-only feature but the lore change of Gorgon Medusa is probably the most famous mythological downgrade of any mythological monster. Originally, Perseus faced the Gorgon by looking at her reflection in his mirrored shield and cutting her head off. Over time, a more pacifist approach was adopted that Medusa could stone herself by looking at her own reflection.
 * Shin Megami Tensei and all its incarnations. Go to www.megamitensei.wikia.com see it in action with various incarnations of demons and mythological figures.
 * Touhou Project has most of its Loads and Loads of Characters taken from Japanese mythology. They all look like little girls and are each capable of Curtain Fire, an ability notably absent in their source material.
 * Many of the monsters from Will Rock have extra powers: the Minotaurs can split in two lesser ones when slain, the Harpies shoot fireballs, Cyclops have goat legs and can eat and spit hot stone pellets and Orthuses can spit fire and acid from their heads.

Web Comics

 * In Sluggy Freelance a succubus is a bus that sucks you up. Seriously.

Western Animation

 * Dante's Inferno: An Animated Epic: The animated short had Cerberus, who was already pretty frightening as a three headed fire breathing dog, re-envisioned as a giant colon like monster with many mouths and residing as a guardian in the third circle of Hell, Gluttony. The gluttonous souls swallowed by Cerberus are forever denied the pleasures they overindulged in while alive. Cerberus's stomach is also the realm that Dante had to cross to enter the next circle of Hell.