Stalkers are enemies from Strife, They are small robots which look like spiders. Stalkers can walk on ceilings. Their melee attack is very effective when combined with the fact that they drop onto you if you are directly under them. They are pretty weak with only 80 Hit Points and can be quickly taken out. They ignore alarms, like all robotic enemies.
It is very possible that the developers intended to expand on these enemies with different variations. This is proven by the fact that additional unused sprite textures were found within the game showing a full 8 rotations of a stalker with bullet attacks. These can be seen here on a fellow community's website.