Crobat, the evolved form of Golbat. With its two additional wings, Crobat is able to fly much faster than Golbat.
– Ash’s Pokedex
I’m not sure why Crobat evolves by Happiness, it doesn’t look very happy, in fact, its face looks rather grumpy. Despite its Pokedex entries saying its feet turned into wings, you can still see feet in its back sprites (and front sprites of Black/White), yet it has no footprint. Crobat’s colouring is the opposite of Golbat’s (ie its body and wing colours are reversed), this is true for the shiny colour as well.
In the anime, Brock’s Golbat evolved into a Crobat.
It’s the first Pokemon in the National Dex order to have a cross-region (and cross-generation) evolution. Crobat’s typing Poison/Flying is unique to its family. Its type combination comes with 4 weaknesses, but it also has 3 double resistances (Fighting, Grass, Bug).
Crobat’s best asset is its Speed. With 130 base Speed, it’s the fastest Poison type (and one of the fastest Pokemon). Its Attack and Defenses are pretty decent as well. It also has the highest base stat total of Poison types (535), and it is the only Pokemon with the highest base stat total of a type that’s not legendary.