Stalker
The Stalker is a predatory life form hostile to the player, other life forms and even sometimes even other Stalkers . They are animals, belonging to the Fauna category.
Stalkers can be found in multiple Biomes, predominantly in the Kelp Forest. If sliced with a Knife they drop a Material: Carbon.
They appear to have developed an interesting capability: Stalkers seem to be fond of Scrap Metal. They pick fragments up and gather them in one spot. This can lead to dangerous situations for an unaware player who finds such a treasure.
Another interesting note is that since [Checkin # 10171 (2014-12-03 00:53:06)] the player is able to lure a Stalker and feed him by hand. To do so catch a Fish and equip it to your hand. You'll be able to then feed a stalker with it.
As of 14.12.2014 the model of the Stalker was changed. Until then the model of the Lava Lizard was used [1]. The Stalker was also listed as Snout Shark in development documents [2].