"The lions are beautiful."
Is there a rule as to when we use masculine-ending(ים) adjectives with feminine-ending(ות) nouns and vice versa?
Some masculine words have a feminine plural form - like in this case, אריה->אריות, or כיסא->כיסאות, or חלון->חלונות; and some feminine words have a masculine plural form, for instance שנה->שנים, ביצה->ביצים, מילה->מילים. These are considered irregular, and adjectives referring to them would all be conjugated according to the word gender. So in the same way that one would say ביצים קשות, (hard-boiled eggs), one would also say האריות יפים. It's rather unfortunate that there isn't a particular rule to these irregular words, so you just have to memorise them.
But אריות are lionesses,
So their adjective should be feminine as well, right ?