are fell beasts as clever as horses or eagles? if they were created in mockery of eagles they would need to have their wits surely?

The winged beasts are possibly as smart as common horses (excluding Shadowfax and his kin), but I doubt that they are as intelligent as the Great Eagles. The Fell Beasts do not seem to be more than animals that have been corrupted and bred by Sauron as mounts for the Ringwraiths.