Hey Luin,
I have thought about this too actually, and after speaking with my guides, this was the painful answer I got.
1) There are only about 4 intelligent species out there. Humans are actually one of the five, but in order of the smartest races you've got Elven, Draconian, Humans, and Mythical (which can be angelic, faery, fae, nymph, sylph, centaur, minotaur, etc. etc.)
2) Mythical is grouped together because they usally live together on mixed realms, and they usually live on realms with higher beings physical present. (Unlike Elven, Draconian and Human)
3) There are 37,000 realms out there, 3 of those are Elven, 2 are Draconian, 7 are Human (what da ya know eh?) and 6 are Mythical. The others are unknown, but they exist within 4,000 separate universes.
4) OMG Physics. This universe has 4 quadrants, Alpha (where we are), Beta (where the Elvens are), Gamma and Delta (where nothing is) and then next to our universe, there's another universe, like a little bubble, with another 4 quadrants, and in that one is Draconians / Dragons, and Raptors / Dinosaurs. The first explains the more intelligent species, the latter is the most common animals. Here on Earth I suppose you could go Human / Dog or Cat.
5) Some souls just won't work in a human body. Elven, Draconian and Mythical souls, tend to work well in human bodies. But the soul of a Carnellian wouldn't work in a human body, since their souls are used to having 14 lungs, filtering toxic air, and breathing hydrogen. Subsequently, the Rock People's souls (I forget their actual name) wouldn't do well in human bodies either because they're so used to excess amounts of force, they'd probably be too rough on the body.
6) So that leaves Elven, Draconian and Mythic to incarnate here, along with regular Human of course.
7) Quick note on vampires though, they are an anti to the physical species and they exist in every race. So basically you could be an Elven and end up a vampire. But then you'd have to be a physical Elven, and travel here, and well be taller than everyone else too, so not likely.
I hope that was enlightening, though you'll have to take my word on most of it.