Well, in some animal species, the inguinal ring (the connective tissue that prevents testes from going back to the abdominal cavity) is open/less restrictive than in humans. So these furry critters can pull their testes inside.
Makes castrating rabbits (for example) a bit difficult!
And testes already have muscles that can pull them closer to the body (but not inside). They’re useful in animals when it gets too cold outside.