Final answer:
The palmaris longus muscle flexes the wrist in humans and may be absent in cats as well as in a percentage of the human population, with little impact on wrist function.
Step-by-step explanation:
The muscle that flexes the wrist in humans but is not found in the cat is the palmaris longus muscle. This muscle provides weak flexion of the hand at the wrist. It is interesting to note that even among humans, the palmaris longus is sometimes absent; lacking this muscle does not typically affect wrist function significantly.