Well, it's not all that different from general differences in species and animals. You have animals that can fly, swim, walk on land, and pokemon aren't really that different and are just sorted into "types." Yuiko here is a pure ghost type, but there's grass, fire, water, rock, ground, flying, electric, dragon, normal, and way more.

So I don't know if you've ever seen a turtle, but Squirtle is a water pokemon and basically is just a turtle able to walk on two legs instead of four.

Or if you've seen a caterpillar, there's a pokemon called caterpie which is literally a giant green caterpillar and goes through the metamorphosis to become Butterfree instead of a butterfly. The difference is, instead of only being a few inches big, Butterfree is at least three feet tall with a wing-span to match.