Maybe not T-Rex specifically, but certainly some kind of reptile. Think about this, though:
What are wings? (In the case of birds) they are arms that evolved large flaps of skin in order to provide lift. So wing = limb, y/y?
Now look at Charizard. Count its limbs. Two legs . . . two arms . . . two wings. Six limbs? Maybe Charizard should be a Bug type!
But in all seriousness, you see this with other Pokemon too. Dragonite's arms, for example, are kind of short and stubby. Looking at other dragons . . . Druddigon - in this case it's the wings that are somewhat vestigial, which is probably why it lacks the Flying type. Latios and Latias - little arms, yeah? And so on, and on (except Salamence, which exists solely to disprove me grr)
My theory is that these Pokemon are all in a state of evolutionary limbo. Whereas in the real world, birds evolved by arms turning into wings (2 arms + 2 legs -> 2 wings + 2 legs), these Pokemon evolve slightly differently, but with the same end result. (ie 2 arms + 2 legs -> 2 arms + 2 wings + 2 legs -> 2 wings + 2 legs) I just think we've caught them halfway, as their arms are shrinking and wings growing.
Also note that Garchomp, one of the very few winged Dragons with fully developed arms, wears its wings on its elbows, thereby supporting this theory in sorts, especially considering that we see those wings grow between the Gabite and Garchomp stages. If there were to be an additional evolution for Garchomp some time in the future (heaven forbid, it's broken enough as it is) we would probably see its forelimbs entirely become wings, appearing more akin to the traditional wyverns of European mythology.
Also, we really need a lung-based Dragon legendary. Gyarados was close, but no cigar. Needs four legs, a beard and a Rain Dance-based set.
