mons

Mons, montis is third declension masculine and an i-stem, so the genitive plural is montium, not montum. The nominative hides the stem: mons is mont- plus -s with the t swallowed, which is why the rest of the paradigm looks like a different word.

The trap is gender. Mens, mors and gens all end in -ns or -rs and are all feminine; mons, pons and fons are masculine, and there is no rule to lean on beyond learning the six as a set. Latin uses mons for anything from a serious peak to a modest hill, so translating it "mountain" every time can make an Italian ridge sound like the Alps. Family: montanus, montuosus, promunturium. English mount, mountain, paramount, amount and surmount all come through French; Spanish monte and French mont keep it plainly.