What type of mammal does this fossil tooth belong to?
It's an entelodont, South Dakota is famous for it's Oligocene mammalian faunas which include entelodonts such as Archaeotherium and Daeodon. 


here is an http://i.ebayimg.com/images/i/181202162847-0-1/s-l1000.jpg for comparison:


Thanks to the guys at fossilforum, they knew in less than a day. this question could have stayed on here for a year so i was impatient.