WordNet sense 1 (a short or shortened tail of certain animals):

bob, dock

WordNet sense 2 (large sheepdog with a profuse shaggy bluish-grey-and-white coat and short tail; believed to trace back to the Roman occupation of Britain):

Old English sheepdog

WordNet sense 3 (having a short or shortened tail):

bobtailed

WordNet sense 4 (having a tail or taillike appendage):

caudate, caudated