The human pregnancy test that uses dead rabbits; the human fertility test that creates human-hamster hybrids; mice with knocked-out genes; and the immortal dog cancer.
Prior to modern pregnancy tests, you could instead use a rabbit, frog, mouse, or toad.
One male infertility test uses golden hamsters, and can actually create a single-cell human-hamster hybrid.
Medical research uses knockout mice to determine the functions of mammal genes.
Dogs worldwide carry canine transmissible venereal tumours. These self-replicating cell lines all originate from a single mutant dog hundreds of years ago.