The Collaborative International Dictionary of English v.0.48:

Coltish \Colt"ish\, a.
   Like a colt; wanton; frisky.
   [1913 Webster]

         He was all coltish, full of ragery.      --Chaucer.
   -- Colt"ish*ly, adv. -- Colt"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coltish
    adj 1: given to merry frolicking; "frolicsome students
           celebrated their graduation with parties and practical
           jokes" [syn: coltish, frolicsome, frolicky,
           rollicking, sportive]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "coltish":
   animated, antic, beardless, boyish, boylike, calflike, capersome,
   childish, childlike, elvish, exuberant, frisky, frolicsome,
   full of beans, gamesome, gay, girlish, girllike, hearty, impish,
   kiddish, kittenish, larkish, lively, maiden, maidenly, mischievous,
   playful, puckish, puerile, puplike, puppyish, puppylike,
   rollicking, rollicksome, rompish, skittish, spirited, sportive,
   sprightly, vital, vivacious, waggish, zestful, zippy