The Collaborative International Dictionary of English v.0.48:

Fatling \Fat"ling\, n. [Fat + -ling.]
   A calf, lamb, kid, or other young animal fattened for
   slaughter; a fat animal; -- said of such animals as are used
   for food.
   [1913 Webster]

         He sacrificed oxen and fatlings.         --2 Sam. vi.
                                                  13.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Fatling
   (1.) A fatted animal for slaughter (2 Sam. 6:13; Isa. 11:6;
   Ezek. 39:18. Comp. Matt. 22:4, where the word used in the
   original, sitistos, means literally "corn-fed;" i.e., installed,
   fat). (2.) Ps. 66:15 (Heb. meah, meaning "marrowy," "fat," a
   species of sheep). (3.) 1 Sam. 15:9 (Heb. mishneh, meaning "the
   second," and hence probably "cattle of a second quality," or
   lambs of the second birth, i.e., autmnal lambs, and therfore of
   less value).