To sign bison, take one or both open '5' hands to either side of the top of your head. Then you curve them up and down as you move your hand(/s) away from your head, and you end up closing your hand(/s) at the end, as if to form the tip(/s) of a bison's horns.
Teach your child the bison sign in the context of seeing bison in story books, cartoons, movies, or natural museums. Better yet, when your toddler can behave during long road trips, why not take them to Yellowstone National Park - or any of the applicable national parks - and let them be in wonder at seeing live bison in grazing action!
This is a useful flash card taught best in conjunction with other animal flash cards, or in preparation for a trip to a natural museum or national park.