While manta rays have been implied to exist since Splatoon 2, with a stage named after a ship, the "Manta Maria," the first time we actually see a manta ray is in Splatoon 3, with the idol character Big Man. Interestingly, he is the first male idol to appear in the Splatoon games. In regard to his appearance, he is overall very flat, similar to real manta rays, although he uses his pelvic fins as two feet he which he is able to stand upright on. He has two eyes with pupils depicted as slits, as well as a mouth resembling a cat's in shape.
Big Man speaks exclusively by saying "ay," with his dialogue being written in parenthesis after. In the Japanese version, instead of "ay," Big Man says "エイ," which is the Japanese word for a ray. This could be a Pokemon sort of situation, where he only actually speaks the name of his species, and inklings are simply able to understand him.
It is unclear exactly how, but Big Man is able to use ink, despite not being an inkling or octoling. This is shown in his boss battle, where he submurges in ink and swims around the arena. During the Gear/Grub/Fun Splatfest, Big Man also mentions being able to swim underwater, implying that he is not hurt by water like inklings and octolings are.