Morgoth's other creatures were many corrupted creatures in Arda.
The Valar left Middle-earth when his power corrupted many of its creatures. They, taking as many plants and animals with them as they could, went to Valinor. Some of these corrupted beasts were hunted by Oromë, who occasionally still went to Middle-earth to hunt, particularly these monsters. While some definitely joined Morgoth, it is unknown how many did. It is unknown how many were sentient, if any. While the Fell Beast almost definitely were these beast, the Watcher in the Water, Nameless Things, and Ungoliant may also have been these beast. Morgoth also made other unnamed creatures that are also in this group of beasts.