* Jaguar
* Jackal
* Jay
* Jellyfish
* Jerboa (a small jumping rodent)
* Jird (a type of desert rodent)
* Jockey (a type of horse specifically bred for racing)
* Javelina (a wild pig found in the southwestern United States and Mexico)
* Junco (a type of small songbird)
Let me know if you'd like more examples!