Toothless of How to Train Your Dragon went from a tiny green dragon which could fit on Hiccup's shoulder in the book series to a Night Fury, decribed as "the unholy offspring of lightning and death itself", and the dragon every viking fears most — rather, the only dragon vikings fear in the film adaptation.
Technically, you could say everyone in Tangled, since the original story doesn't really have any sort of "action." Gothel is a sort of weird example—she becomes more badass by becoming less powerful, going from a witch in the fairy tale to a knife-wielding badass (she knows a spell to retain her youth and is also implied to be able to fly — which is seen in only one scene—, but otherwise she never uses magic in the whole film).
In the original Jungle Book novel, Shere Khan was portrayed as being crippled due to a leg deformity he received during birth and was a rather pathetic Smug Snake to boot. In the Disney adaptation, however, Khan is portrayed as a menacing, bloodthirsty, yet hammy and whimsical predator who is feared by everyone in the Indian jungle and is constantly determined to kill Mowgli for being a human. In the sequel, however, he is flat out menacing.
And in the Soviet animated adaptation, Shere Khan is a scary villain from his very first appearance. Though it goes both ways, as Shere Khan is a lot more cunning and manipulative in the book.
This seems to make the anthropomorphic transition as well, if TaleSpin is of any evidence. In this adaptation, he is a ruthless and extremely savvy businessman (er, businesstiger) that could easily have Baloo hunted down and crushed if not for his moral code.
In the original book of Sleeping Beauty, there was a minor villain known as The Evil Fairy. When the book was adapted into a film, that minor baddie was morphed into Maleficent, one of the most badass Disney villains to date.
Rise of the Guardians lives by this trope. Santa is a former Russian bandit who duel-wields swords and goes by "North", the Easter Bunny is an Awesome Aussie with boomerangs as weapons, the Sandman is the most badass out of all of them, and the Tooth Fairy, while the weakest, can definitely hold her own in a fight. Jack Frost has also been de-aged and is very powerful, while Pitch Black the Boogeyman is capable of taking them all on.
JLA Adventures: Trapped in Time sees Dawnstar get some impressive light powers to fill out her comic's powerset, which was limited to tracking and flight. Then the Time Trapper, who was always a major powerhouse, gets treated like a Lovecraftian horror here. It's a Shout-Out to the Paul Levitz/Keith Giffen Legion run where he was in his Entropy Personified incarnation.