Sadly, the De formis survives in only 1 manuscript, that is heavily damaged, and the scribe is the same one from the De esse intelligibili, who is an extremely poor copyist. Thus this may well be the most challenging entry in the Petri Thomae opera.
Here is Peter's description of form from the beginning of the work, after he has surveyed the definitions of Aristotle, Averroes, Augustine, and Avicenna. Following the definition he breaks it down word by word in true medieval style and offers commentary on it.
Quaestiones de formis, q. 1 a. 2:
forma est pars essentialis compositi, alterius eiusdem partis actuativa simpliciter, ab eo tamen dependens in fieri et in esse, vel in esse tantum vel compositi, principaliter essentiativa vel specificativa.
Form is an essential part of the composite, absolutely actuating the other part of the composite, yet depending upon it both in being and in becoming, or in the being alone of the composite, essentiating and specifying [the composite].