The Collaborative International Dictionary of English v.0.48:

Fantastical \Fan*tas"tic*al\, a.
   Fanciful; unreal; whimsical; capricious; fantastic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fantastical
    adj 1: existing in fancy only; "fantastic figures with bulbous
           heads the circumference of a bushel"- Nathaniel Hawthorne
           [syn: fantastic, fantastical]
    2: ludicrously odd; "Hamlet's assumed antic disposition";
       "fantastic Halloween costumes"; "a grotesque reflection in
       the mirror" [syn: antic, fantastic, fantastical,
       grotesque]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "fantastical":
   Barmecidal, Barmecide, Gothic, absurd, airy, antic, appalling,
   apparent, apparitional, arbitrary, astonishing, autistic, baroque,
   beguiling, bewildering, beyond belief, bizarre, brain-born,
   capricious, chimeric, cockamamie, concocted, conspicuous,
   cooked-up, cranky, crazy, crotchety, deceptive, delusional,
   delusionary, delusive, delusory, dereistic, dream-built, dreamlike,
   dreamy, egregious, enigmatic, erroneous, exceptional,
   extraordinary, extravagant, fabricated, fabulous, fallacious,
   false, fancied, fanciful, fancy-born, fancy-built, fancy-woven,
   fantasied, fantasque, fantastic, fascinating, fictional,
   fictitious, figmental, flaky, florid, foolish, forged, formidable,
   freakish, grotesque, harebrained, hatched, high-flown, humorsome,
   illusional, illusionary, illusive, illusory, imaginary, impossible,
   incomprehensible, inconceivable, incredible, invented, kinky,
   laughable, legendary, ludicrous, made-up, maggoty, manufactured,
   marked, marvelous, miraculous, misleading, monstrous, moody,
   motiveless, mythical, nonsensical, notable, noteworthy, noticeable,
   notional, of mark, ostensible, outlandish, outrageous, outre,
   outstanding, passing strange, petulant, phantasmagoric, phantasmal,
   phantom, phenomenal, poppycockish, preposterous, prodigious,
   put-up, puzzling, quirky, rare, remarkable, ridiculous, rococo,
   seeming, self-deceptive, self-deluding, sensational, signal,
   specious, spectral, strange, striking, stupendous, superior,
   supposititious, temperamental, trumped-up, unaccountable, unactual,
   unbelievable, uncommon, unexpected, unfounded, unheard-of,
   unimaginable, unique, unprecedented, unreal, unreasonable,
   unrestrained, unsubstantial, vagarious, vagrant, visionary, wanton,
   wayward, weird, whimsical, wild, wonderful, wondrous