There is a common misconception that elephants are the only animals that have 4 knees, but elephants are the same as all other quadruped mammals, like horses and dogs, in that they have knees only in the back legs. Skeletal images clearly depict that the front leg joint bends in the same way as human elbows rather than knees. In animals like the horse, the front joint that looks like a knee is more appropriately compared to a human wrist, making the long lower leg more comparable to the third metacarpal of a middle finger. All mammals' skeletal structures are very similar due to the fact that they descend from common ancestors.
In addition, all tetrapods, which are all vertebrates with 4 limbs, and those animals whose ancestors had 4 limbs like snakes are also based on similar structures. These common limbs all share 2 long bones that combine tarsal bones in the back and carpal bones in the front. All of these limbs also have 5 digits. Even in animals that have less than 5 digits, the 5 digits are present in embryonic state.Learn more about Zoology