Pidgeot (R/B):
When hunting, it skims the surface of water at high speed to pick off unwary prey such as Magikarp.
I always thought that was interesting because it proved that Pokemon do eat each other.
Darumaka (White):
Darumaka's droppings are hot, so people used to put them in their clothes to keep themselves warm
My ancestors used to put poop in their clothes too!
Next here's a pair that are really creepy. Naturally they're both ghost types,
Haunter (SoulSilver)
Its tongue is made of gas. If licked, its victim starts shaking constantly until death eventually comes.
Yamask (Black):
Each of them carries a mask that used to be its face when it was human. Sometimes they look at it and cry.
Both of these are pretty scary, but Yamask's really creeps me out. I find it awkward raising and training something that is officially stated to have once been human. This is worse than the Gastly/ Cloyster theories because it's real.
Magikarp is an interesting case (when it's not being eaten by Pidgeot that is) because it has two entries that contradict each other.
The first is from Heartgold;
An underpowered, pathetic Pokémon. It may jump high on rare occasions, but never more than seven feet.
And this is from Platinum:
A Magikarp living for many years can leap a mountain using Splash. The move remains useless, though.