I really enjoyed going in the winter, but it might not be for everyone. Much of the park isn’t accessible unless you take a snow coach tour or rent a snowmobile. The only area open to road traffic in the winter is the northern entrance in Gardiner to Mammoth Hot Springs, and then the road through Lamar Valley that connects to the northeast entrance at Silver Gate/Cooke City.
That said it was absolutely beautiful, it was by far my favorite vacation I’ve taken. It was COLD though. One day it hit -25f with a high of 2f that day haha
Edit: I misread your comment, you probably don’t need my explanation if you go every year 😅
Bison! And yes there are still some in the US. Not as many as there used to be in the 1800’s, but there are still quite a number of them around the country.