If it was actually owned by someone which it looks like it was then yes its illegal. if it was wild and you had proper hunting permits and if your allowed to hunt that type of animal then it would not be illegal to kill an animal just because of a birth defect. It looks like Buffalo hunting is legal in Texas so it would come down to ownership of the animal.
Are anyone other than Lakota allowed to hunt on Lakota land? Was the animal ON Lakota land?