Those Nutty Squirrels

These guys are Skidid (a long story and combination, but: Sciridae + Skid + Splendid). Yes, that's right: they are all Skidid.

From left to right, you have
Skidid West : Fire/ground
Skidid North: Dark/ground
Skidid South: Grass/ground
Skidid East: Ice/ground

aside from their colors and types, they do have some more differences. Each of them has longer hair in certain spots: the red has a faux-hawk (he stole that from me, I swear.
The silver (you can't see it) has more hair on his tail (just like me... oh wait, wrong tail). also, their ears are shaped differently. and notice that the black one is the only one without blue eyes.

They all evolve into their next stage at level 21. And a spoiler, the next stages lose the ground and gain a flying.

and that's all.

Skidid (c) EpicDay
Pokemon (c) Nintendo