The Collaborative International Dictionary of English v.0.48:

Bestiality \Bes*tial"i*ty\, n. [F. bestialit['e].]
   1. The state or quality of being bestial.
      [1913 Webster]

   2. Unnatural connection with a beast.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bestiality
    n 1: the stupid brutal quality of a beast
    2: sexual activity between a person and an animal [syn:
       bestiality, zooerastia, zooerasty]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "bestiality":
   Adam, Gothicism, Neanderthalism, abominableness, anal intercourse,
   animalism, animality, atrociousness, autoeroticism, awfulness,
   barbarism, barbarity, barbarousness, baseness, beastliness,
   bloodiness, bloodlust, bloodthirst, bloodthirstiness,
   bloody-mindedness, brutality, brutalness, brutishness, buggery,
   cannibalism, carnal nature, carnal-mindedness, carnality,
   coarseness, contemptibleness, cruelness, cruelty, cunnilingus,
   despicableness, detestableness, direness, disgustingness,
   dreadfulness, earthiness, egregiousness, fallen nature,
   fallen state, fellatio, fellation, ferociousness, ferocity,
   fiendishness, fierceness, filth, flesh, fleshliness, foulness,
   fulsomeness, grossness, hand job, hatefulness, heinousness,
   horribleness, ill breeding, impoliteness, incivility, infamousness,
   inhumaneness, inhumanity, irrumation, lapsed state, loathsomeness,
   lousiness, manipulation, masturbation, materialism, nastiness,
   nefariousness, noisomeness, nonspirituality, notoriousness,
   obnoxiousness, odiousness, offensiveness, onanism,
   oral-genital stimulation, outrageousness, pederasty, philistinism,
   postlapsarian state, rankness, repulsiveness, rottenness,
   ruthlessness, sadism, sadistic cruelty, sanguineousness, savagery,
   savagism, scandalousness, scurviness, self-abuse, shabbiness,
   shamefulness, shoddiness, sodomy, sordidness, squalidness, squalor,
   swinishness, terribleness, the Old Adam, the beast, the flesh,
   the offending Adam, the pits, troglodytism, truculence,
   uncivilizedness, uncleanness, uncouthness, uncultivatedness,
   uncultivation, unculturedness, unrefinement, unspirituality,
   vandalism, viciousness, vileness, violence, wanton cruelty,
   wildness, worthlessness, wretchedness