What makes a pokemon be able to use certain moves?

Okay, so there are obvious ones like Tail Glow (needs a tail) and Steel Wing (needs wings) and Pound (needs some sort of tail), but what about moves like Swagger? Flatter? Surf? Mud Slap? Swift?

Does Nasty Plot have some sort of IQ requirement, so that the pokemon can actually have the intelligence to plot out something nasty? Why can't most pokemon learn Bite when most pokemon have teeth? Do you need to be fat enough to shake the earth for Earthquake? And what exactly do you need to be able to call all of these flying rocks from the sky (Rock Tomb, Rock Slide, Stone Edge, Rock Throw)?

I've never understood why Nidoran M can learn Peck. It doesn't have a beak.

However, this thread made me lol. You'd have to be pretty fat to shake the earth.

It has a horn. All it has to do is make the pecking motion, and it should be virtually the same effect. A nidoran's peck looks more painful than, say...a piplup's.