Something found in a pen or octopus

INK

This dark liquid was historically made from squid sacs and is still used to sign important documents today.

This clue plays on the double meaning of "pen"—it can refer to a writing tool or an enclosure for animals. An octopus naturally produces ink as a defense mechanism, while a pen (the writing kind) contains ink for writing. The answer ties both interpretations together neatly with a single word.