This seems like something obvious that I should really know, but I worry that I'm forgetting something.

Squids, octopuses, and cuttlefish all squirt ink. (Nautilus, the throwback cephalopod, does not.) The sea hare, a particularly large instantiation of sea slugs, also squirts ink.

Does any other animal in the whole world produce ink?

Bombardier beetles spray aerosolized toxins. Texas horned lizards shoot blood out of their eyes.

But I think cephalopods and sea hares are the only inkers!