Yes, they do have T-poses. The way I understand it, anything that is attached to the main body of a Pokemon with a joint is extended out, so things like arms, legs, ears, tails, etc. will be as straight as they can be. This doesn't apply in situations where these parts do not move, for example, Dragonite's antennae are not stretched out completely because, to my knowledge, at least, they do not move, and are basically just a static part of Dragonite's head. There are also instances like Klefki's, where part of the Pokemon is detached from the model of the Pokemon in the T-pose. You'll have to look for yourself if there are any other instances your curious about, so I'll leave links for T-poses of the models from each Pokemon game below. Also, here are the T-poses for Tauros and Rayquaza. I couldn't find Cherrim on that website. I'd also like to share Beautifly's and Exeggutor's models.
X/Y Models
ORAS Models
Sun/Moon Models
USUM Models
Sword/Shield Models
The models in those links are user-submitted, and all of them are incomplete, especially those of the more recent games. The majority of the models in X/Y are there, however, as with the other ones, there are still some that are missing.
