WordNet (r) 3.0 (2006):

horse around
    v 1: indulge in horseplay; "Enough horsing around--let's get
         back to work!"; "The bored children were fooling about"
         [syn: horse around, arse around, fool around, fool]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "horse around":
   act up, antic, be foolish, be stupid, behave ill, caper, caracole,
   carry on, cavort, clown, clown around, coquet, curvet, cut a dido,
   cut capers, cut up, cut up rough, dabble, dally, dance, disport,
   fiddle, fiddle with, fiddle-faddle, fidget with, finger with,
   flirt, flounce, fool, fool around, fool with, fribble, frisk,
   frivol, frolic, gambol, get funny, get into mischief, go haywire,
   go on, hell around, idle, invite ridicule, jerk off, kid around,
   loiter, maffick, mess around, misbehave, misdemean, monkey,
   monkey around, piddle, play, play around, play the buffoon,
   play the fool, play with, potter, putter, raise Cain, raise hell,
   raise sand, raise the deuce, raise the devil, raise the roof,
   rollick, romp, roughhouse, skip, smatter, sport, tinker, tomfool,
   toy, toy with, trifle, trip, twiddle, whoop it up