Revert model to t-pose I'm aware of the "Rest Position" button, but I'm looking to somehow get the model to a T-pose in "Pose Position" (because you can't pose the model if it's in "Rest Position" mode), so I can pose it from there. I downloaded a rigged model and it wasn't in T-pose, but I want to revert it to a T-pose to make it easier to pose the model.

I'm hoping there's a button because I can't find some of the bones

Wow that's a lot of pose hope it's not too unclear
To return to the original pose, simply reset all the transformations. 

Make sure your are in Pose Mode, by selecting the Armature and choosing Pose Mode in the 3D View (CtrlTab).

Then select all bones (A) and press


AltG to reset location
AltR to reset rotation
AltS to reset scale