It wouldn't surprise me, given how they generally are. Then again, the natives seem a bit inconsistent...
I've always assumed that fairly reasonable limitations apply, honestly. For example, I'd expect my 1.6 meter tall Noctowl to be able to carry me, given both her size and the fact that bird Pokemon are probably stronger and less subject to conventional rules of flight than an actual bird would be. However, I really doubt you'd be able to get anywhere using a Pidgey or something similar.
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I've always assumed that fairly reasonable limitations apply, honestly. For example, I'd expect my 1.6 meter tall Noctowl to be able to carry me, given both her size and the fact that bird Pokemon are probably stronger and less subject to conventional rules of flight than an actual bird would be. However, I really doubt you'd be able to get anywhere using a Pidgey or something similar.