When a creature doesn't have an external scrotum, but instead the testes are inside the body. This is the normal condition for reptiles, birds, and fish, as well as some mammals like elephants and cetaceans.

This tag should only be used when the testes being inside the body is visible somehow, usually through an internal shot or a diagram.
If there is nothing making the internal state of the testicles visible, consider if it meets the criteria for sackless instead.