I am not an expert on animal intelligence, but octopuses seem as or more intelligent than monkeys from my limited understanding. They aren’t proven as intelligent as the great apes—none have been taught a human language that I know of—but might have languages of their own, and considering their other feats I would be mildly surprised if there aren’t at least several species which could learn a sign language which doesn’t take hands.
https://en.wikipedia.org/wiki/Cephalopod_intelligence
I am not an expert on animal intelligence, but octopuses seem as or more intelligent than monkeys from my limited understanding. They aren’t proven as intelligent as the great apes—none have been taught a human language that I know of—but might have languages of their own, and considering their other feats I would be mildly surprised if there aren’t at least several species which could learn a sign language which doesn’t take hands.