Lurkers are an animal that can only be found at the Vasagatan story location. Lurkers are one of the only two animals in Raft that can respawn when the player gets a certain distance away from their spawn location. There are five lurkers on the Vasagatan, one spawns on each floor except for the top floor.
Lurkers have two different attacks, they can do a jump at you if you are far away from them, or they can swipe at you if you are close to them. The easiest way to kill them is to stand close to them and circle strafe around them while stabbing with your spear. Lurkers can be killed with 3 stabs from a metal spear.
I recommend getting to the Vasagatan and farming all 50 lurkers while you are there so that you don't have to come back. You can kill each floor's lurker as you ascend the boat and then jump outside and back down to the bottom. The distance it takes to go up the boat and then back down should be enough for the lurkers to respawn by the time you kill a set of five.