Myotragus
(Varies by Species)

Living on the Balearic Islands of the Mediterranean until around 3,000 years ago. To survive on islands with extremely limited food, it did something no other mammal is known to have done — it became effectively cold-blooded, slowing its metabolism to reptile-like levels and growing at variable rates depending on food availability, just like a lizard or crocodile rather than a warm-blooded mammal. 

 

It also had forward-facing eyes and rodent-like ever-growing teeth to complete its thoroughly un-goat-like biology. It went extinct shortly after humans arrived on the islands!