No, Voldemort was not an Animagus. He didn't see any reason to become an Animagus despite being a gifted wizard and knowing a lot of magic. This may simply be because his sole purpose was to obtain immortality and become the strongest wizard who could control the world around him. Being an Animagus wouldn't grant him any otherworldly powers and most of the powers he wanted, he already had. He used Horcruxes to preserve his soul and was also very arrogant to see any loopholes in his plan.