Lord Vishnu has many types of avataras. In fact, His avatars are innumerable:
O brāhmaṇas, the incarnations of the Lord are innumerable, like
rivulets flowing from inexhaustible sources of water. [SB - 1.3.26]
But these ten are only the prominent ones; especially because they take place on this material plane, our earth.
avatarati it avatara - so those are called avataras because they descend to earth and take up material bodies of transcendental nature. Where as many other avatars take place on other planes of existences, just like the Mohini one. Hence, Mohini is not in the list of the ten avatars.
Shrimad Bhagavatam 1.3
A very good article to understand about the avatars