There were giant hyenas in North America.
Heck, EVERYTHING was giant—even birds that stalked antelope!
After the 100 year long comet strikes of 10,900 BC and the resulting firestorms that burned off 4% of Earth’s vegetation, the oxygen level fell below what those large beasts needed to live. The strikes also ushered in the Younger Dryas Event, the coldest period in recent geological history, which didn’t help their survival.