It is a mammal which is why it's so fascinating!

The goat bones have a growth pattern only found in reptiles suggesting it had adapted to varying availability of food. Other morpholigic changes (smaller brain, smaller eyes) also suggest that it had at a minimum evolved to withstand low nutrition periods.

The working theory to these observed structures is that this mammal a rare example of a warm blooded creature turning cold blooded.

Stranger things have happened though, the evolutionary path of whales is particularly strange.