The Collaborative International Dictionary of English v.0.48:

Bobtail \Bob"tail`\, n. [Bob + tail.]
   An animal (as a horse or dog) with a short tail.
   [1913 Webster]

   Rag, tag, and bobtail, the rabble.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bobtail \Bob"tail`\, a.
   Bobtailed. "Bobtail cur." --Marryat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bobtail
    adj 1: having a short or shortened tail; "bobtail mare" [syn:
           bobtail, bobtailed]
    n 1: a short or shortened tail of certain animals [syn:
         bobtail, bob, dock]
    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 [syn: Old English sheepdog,
       bobtail]