The Collaborative International Dictionary of English v.0.48:

pet \pet\, a.
   Petted; indulged; favorite; admired; cherished; as, a pet
   child; a pet lamb; a pet theory; a pet animal.
   [1913 Webster]

         Some young lady's pet curate.            --F. Harrison.
   [1913 Webster]

   Pet cock. [Perh. for petty cock.] (Mach.) A little faucet
      in a water pipe or pump, to let air out, or at the end of
      a steam cylinder, to drain it. [Also spelled petcock.]
      [1913 Webster]