The spontaneous tumor biology has been investigated with the support of animalists using animals as a preclinical model allowing translation of results in clinical practice. This review provides an insight into the field of comparative oncology. Evidence shows that companion animal health care is impressively growing in terms of development of new therapies and diagnostic tools, nutrition and disease prevention. However, even if most animal tumors might be a reliable model to study human carcinomas, many open questions, related to the opportunities to select and recruit new models in oncology, along with their legal and ethical implications, remain unanswered.