Extraterrestrial Pokemon?

Every so often, creatures would land on earth, which looked cute and wildly unique and cartoonish, not unlike Pokemon (but vastly different). They all had a single eye, or in some cases no eyes. They had the intelligence level of a house cat.

Humans would adopt these creatures as pets, feed and nurture them. It was a symbiotic relationship. Another creature landed one day, this one had 2 eyes, Grey fur, sharp teeth, spoke human language, and a bloodlust.

The creature singlehandedly wiped out the Humans. The Earth was now populated by this new invasive species.