1. Rabbit, ferret, gerbil, turtle
2. Rabbit, gerbil, turtle, ferret
3. Rabbit, turtle, ferret, gerbil
4. Rabbit, ferret, turtle, gerbil
5. Rabbit, gerbil, ferret, turtle
6. Rabbit, turtle, gerbil, ferret
Those 6 combinations times 4 animals mean that there should be 24 different orders Kevin can feed his pets.