Thanks Jeffrey, Eileen, Jonathan, Holly, and Anna for your excellent comments below. I'm in the midst of an editing marathon right now, but to let you know I'm still alive, I'll share this.
In "Les Cynocéphales: Étude d'une tradition tératologique de l'Antiquité au XIIe s” (Cahiers de civilisation médiévale 24 (1981): 117-29, at 123, Claude Lecouteux speaks about how the question of the necessity of baptism made monsters a theological topic. For monsters born from bestiality, baptism was generally required:
Ainsi dit l'ancien rituel romain suivi par de nombreux rituels provinciaux. Toutefois, certain théologiens, s'appuyant sur Aristote, distinguaient si un homme or une femme avait eu commerce avec une bête; dans le premier cas, le monstre issue d'un tel accouplement devait être baptisé sous condition car c'était peut-être un homme; il ne pouvait l'être dans le second cas, car il n'en était certainement pas un. Depuis qu'on ne croit plus à la fécondité de telles unions, le Droit Canon a été modifié sur ce point.He cites Lucius Ferraris, Bibliotheca canonica, ed. Bucceroni (Rome, 1885), volume I, p. 499. Unfortunately, I'm having some trouble tracking down the appropriate passage. However, thanks to the dubious gift of Google Books, I did find this, which speaks of a certain "Tractatus de Baptismo," which considers a "monstrum genitum ex muliere et bruto, tum etiam ex viro et bruto femella, quod Auctor ibi possibile ponit, per nos impossibile praedicari." There's science again, stepping in our fun. Since my school skimps on research money, and since the online PL has recently stopped being useful (the baleful hand of Migne chills us even now), and since I'm honestly too busy to write this post, I can't track this down any further, at least right now (although Suzanne Magnanini's treatment of monstrous generation might be useful). But maybe you know something? Or maybe you're just amused by an odd legal tidbit.
This is what's said by the ancient Roman ritual, followed by many provincial rituals. However, certain theologians, relying on Aristotle, distinguished between whether a man or a woman had had "commerce" with a beast; in the first case, the monster issued from such a coupling should be baptized conditionally because it was perhaps a human; it couldn't be so in the second case, as it was certainly not human. Since the fertility of such unions is no longer believed in, Canon Law has been modified on this point.