Homo homini lupus

The bitter observation "homo homini lupus" translates into English as "Man is a wolf to man."

The pattern in Latin is the same as in asinus asino et sus sui pulcher - the noun first in the nominative (ie subject) case and then in the dative (ie indirect object) case.  Homo, hominis is, of course, a third declension noun same as sus, hence the dative with an -i ending.

Sus sui pulcher

Sus sui pulcher is part of a pithy Latin proverb: "Asinus asino et sus sui pulcher (est)."  The full phrase means: "A donkey (is) beautiful to a donkey, and a pig to a pig."

Asino and sui are the dative singulars of the second declension noun - asinus, -i donkey and the third declension noun sus, suis pig.