« pris » est le participe passé de « prendre », pas de « pendre ».

Cependant, on pourrait utiliser la participe passé de pendre ici :

Je me frottai les yeux, je regardai attentivement, je le vis pendu; je tombai en faiblesse.

Pangloss didn’t hang himself, but he did simply "hang". Pendre has both a transitive variant and an intransitive variant, as noted in the Trésor de la langue française:

I. − Empl. trans. et pronom.

II. − Empl. intrans. Qqc. pend (à, après, sur qqc.).

So je le vis pendre is equivalent to:

Je le vis qui pendait.

Je le vis pendant.

Incidentally, English also has this double status of hang (e.g. "The criminals will hang at daybreak"), so we needn’t find it too unfamiliar.

Also, it goes without saying that since there is also that transitive variant you already know well, pendu would be equally grammatical.