Yes, these semi 'furred' animals do molt. Even sheep molt if you don't shear them (eventually). I would expect it is about that time--they should be 'slicker' closer to Fall--and then grow another coat again, of sorts. Not ever having touched a bison, I don't know about the texture, but 'haired' sheep look like this part of the year--they have a coat of sorts and shed or molt. This has obviously matted and the mats are falling away since they are 'dead hair'.