Clifford D. Simak 
Werewolf Principle 

In the middle-distant future, Andrew Blake, discovered on a distant planet huddled inside a capsule, is brought back to Earth suffering from total amnesia. Over 200 years old, he thinks and acts like a man but becomes frighteningly aware of two alien beings that lurk within his body – a strange biological computer and a wolf-like animal. With the latter in control he breaks out of hospital to look for his past.