The following is based on Fallout Tactics and some details might contradict canon.
The crab-like scurry robot is a utility robot model that was most likely designed before the Great War. After the War, it was used by the Calculator, the mad cyborg from Vault 0, and adapted for combat.
Like many smaller robots, Scurry robots can be seen in a variety of locations. They patrol most open ground in robot-controlled areas. They have the ability to hide under the dirt and pop out at unsuspecting victims and almost always appear in groups. Due to relying almost exclusively on ambush tactics, they prefer to attack up close and do not possess any ranged attacks.
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