The word unicorn comes from Latin, the sources being uni- "one", and cornu "horn".
Although English has dropped the final u from what would have been unicornu (plural unicornua), that letter survives in cornucopia "horn of plenty" (compare English copious "plentiful"). Other omissions of u are found in English (from French) cornet "little horn" and tricorn hat.
In English are found the adjectives cornual and corneous "hornlike", and cornigerous "having horn(s)"; and the diminutive noun corniculum used in anatomy for parts resembling a horn in shape.
Unrelated, however, to cornu and one another are the edible grain corn, the cornel tree, and the hooded crow Corvus cornix.
Unicorn has a Greek synomym μονοκερας (in modern Greek more likely μονοκερως), most directly transliterated as monokeras "one horn", but more frequently seen in English as the Latinized monoceros.
Well known is the prefix mono- "one". Among English derivatives of keras are keratin, the zoologists' term for the material of which horns and claws are made; and the medical term keratitis, inflammation of the cornea. Meanwhile, ceros is found in rhinoceros "nose horn" (compare rhinoplasty "nose job").
It is not unusual to have in English both the unLatinized and Latinized forms of a Greek word. An example is the unLatinized physics term kinematic "pertaining to motion" and its Latinized counterpart cinematic "pertaining to motion pictures".
The classically-formed plural of monokeras is monokerata, and of monoceros is monocerotes, pronounced with five syllables.
Latinized or not, monokeras and monoceros come from the third declension; hence there is no precedent to write the plural of monoceros as monoceri, and similarly without basis is the adjective monocerous — those would be for a second declension noun. Completely separate is cornu, which comes from the Latin fourth declension, scarcely related to the third.
A classically-formed adjective would be monocerotic (similar to eros–erotic or rhinoceros–rhinocerotic). This leads to such words as the nouns monoceroticism (although the shorter form monocerotism can be justified) and monoceroticist, the verb monoceroticize, and the adverb monocerotically.
When bringing nouns from Greek or Latin into English we ordinarily adopt the nominative singular, and often the nominative plural (for example, curriculum, curricula). However, astronomers employ the genitive singular for naming stars. For instance, the brightest star in the constellation Monoceros is Alpha Monocerotis, the second-brightest Beta Monocerotis, and on through the Greek alphabet.
It is difficult to find an adjective of monokeras that is attested in English, but monokeratic is a suggestion.