Lionrat

Lionrat is a future descendants of Gambian pouched rats that were accidentally introduced to North America by the now extinct humans. They have a mane like a male lion, hence their name, but unlike the lion's mane, their mane is poisonous due to the lionrats feeding on native descendants of invasive poisonous plants, to help protect themselves against predators. They are much bigger than their ancestors, about the size of a capybara, but is much longer than a capybara due to its long tail.

Evolution
After the Late Holocene ended, humans are now gone, but because of this, they had failed to eradicate Gambian pouched rats from North America. Their numbers grew due to lack of exterminators and less predators around. They eventually grew much bigger and became more omnivorous than their ancestors. They became fully terrestrial due to their large size. They started to grow poisonous manes as protection against predators. Gambian pouched rats had evolved into lionrats.