In Renaissance Italian the word mostro still echoes the Latin monstrum, i.e. something amazing, something exaggerated, not necessarily a "monster" in our current sense. In the section of Il Mondo Creato devoted to this subject, Tasso starts from a brief remark by Dante, Inferno 31: 49-57, but develops it much more in depth. In fact, in the crystalline worldview of the Late Middle Ages, they tended to water down everything that put rules at risk, while Renaissance authors were interested precisely in exceptions, in the puzzling complexity of the universe, therefore studying even natural phenomena that would shock Medieval eyes and minds, like deformed fetuses, etc.
As to monsters in the strictest sense, including those of classical mythology, Tasso introduces -- or adopts -- an interesting explanation: Hybrids like the Minotaur, or critters with an extra number of limbs like the dragon in the Book of Revelation, were suggested by Nature because of the many freaks "she" gives birth to. Sort of an inverted Euhemerism, as well as a sign that the cosmos was no longer seen as a perfect clockwork. To the extent that Tasso sometimes shows a proclivity for, at least, a half-Gnostic pattern of Nature.