Pikachu is, like every other thing in Pokemon, based off of something in real life. The Pika mouse. http://faculty.ucr.edu/~chappell/INW/mammals/pika5.jpg It's something most people don't really pay attention to; they know it's a rodent, but don't actually know it's based off a real-life rodent.
Something which was added in either 4th or 5th gen, which not many people realize, is that Metapod now has the move Bug Bite. I kid you not. A formerly-useless Pokemon now has the ultimate hold-out. Also, added in Gen 5, Magikarp can learn Bounce, a flying-type move. God knows why you'd want it, but it's there.
Some other ones are kind of obvious, but... Slugma = Slug + Magma, Magcargo = Magma + Escargot (Fried snails). Oh yeah. Something else. You know that bag that Delibird carries? It feeds its young with that bag. "It nests at the edge of sharp cliffs. It spends all day carrying food to its awaiting chicks." If you catch Delibird, YOU ARE KILLING ITS YOUNG AND YOU SHOULD BE ASHAMED.
Also, Spoink (That weird, spring pig thing) keeps its heart rate up by bouncing. If it stops bouncing by, say, Stun Spore or sleep, it'll die. "Spoink bounces around on its tail. The shock of its bouncing makes its heart pump. As a result, this Pokémon cannot afford to stop bouncing - if it stops, its heart will stop." The lesson here is, if your opponent has a Spoink, close the game down and try to avoid them. Or commit Pokemon homicide. Whichever.