Cave lurker

The Cave lurker is a monster fought during the The Grand Opening quest. Players have to kill it to obtain its hide to make a Spiked cape for Lestri the Dorgeshuun fisherman. After obtaining the hide, you cannot attack the lurker again, unless the hide or the cape is lost (giving it to Lestri doesn't count). To find the lurker, you must go south of Dorgesh-kaan, then go left and the lurker should spawn. Killing it is relatively easy, but lower-levelled players should take caution.

Trivia
It closely resembles a Spirit dagannoth, although it is smaller and its eyes are a more yellow-greenish color, as opposed to the familiar, whose eyes are a darker yellow.