Second Thymus
The thymus is also present in most vertebrates, with similar structure and function as the human thymus. Some animals have multiple secondary (smaller) thymi in the neck; this phenomenon has been reported for mice and also occurs in 5 out of 6 human fetuses. As in humans, the Guinea pig's thymus naturally atrophies as the animal reaches adulthood, but in the athymic hairless guinea pig (which arose from a spontaneous laboratory mutation) possessed no thymic tissue whatsoever, and the organ cavity is replaced with cystic spaces.
Read more about this topic: Thymus