Monotonous rhymes with:
chitinous, gelatinous, gluttonous, mountainous, mutinous, tetanus, titinus, androgynous, asynchronous, bituminous, brazenness, chitinous, deaconess, exogenous, gelatinous, givenness, gluttonous, homogenous, indigenous, larcenous, luminous, membranous, mountainous, mutinous, nitrogenous, nonpoisonous, oleaginous, ominous, outspokenness, patroness, poisonous, polygynous, ravenous, ruinous, tetanus, titinus, uranus, villainous, voluminous
Monotonous sounds like:
maddening, madding, madness, madonna's, maidens, maintaining, maintains, maintenance, mandamus, mandating, manhattan's, matinees, mating, matting, meat-eating, medianews, mediating, meditating, meditations, mediums, mednick, meeting, meeting's, meetings, mending, mendonca, mendonsa, mentink, mentioning, mentions, methanex, mindedness, minding, minnetonka, minting, mitnick, mittens, modems, monadnock, monatomic, montana's, montanans, montanez, motioning, motions, motown's, mountain's, mountainous, mountains, mounting, mouthing, muddying, munitions, mutating, mutations, muting, mutinies, mutinous