We have maintained huge megafauna populations though, who are ready and able to take over the moment we go. Cows, sheep, and yes, horses like shown in the comic, are prime examples. We’re also doing a damn good job of killing all their natural predators, namely wolves and big cats.
Horses have actually become an invasive species in some parts of the Americas and driving out native large herbivores. Ever heard of American wild horses? They’re technically “feral horses” because they did not exist in this hemisphere before Europeans came.