The Crab Synth is an incomplete Half-Life 2 enemy only glimpsed towards the end of the game in the Citadel on a conveyor belt along with the Mortar Synth, during Gordon Freeman's second pod ride. It is unknown where it is transported to, as it is never encountered in combat anywhere during the game or its episodes. With the Mortar Synth, it is said to be "the backbone of the Combine's subsequent armies."
The Crab Synth has many spiked protrusions and various mechanical systems on its front and rear. According to its model animations, it can charge at targets like the Antlion Guard, but with likely even more damage due to its size. It also uses a Gatling gun-like weapon, but only when walking (as seen in its walking animation and concept art). When not walking, it is stored under its belly, hidden behind a plate.
Unlike the Mortar Synth, the Crab Synth can be hurt and killed if spawned. Upon contact, red blood will appear, in the same manner as with the Half-Life 2 version of the Ichthyosaur, as they are incomplete NPCs, not to come in direct contact with the player during normal gameplay.
Behind the scenes
The Crab Synth was never completely finished and seems to bear a temporary name, likely related to its similarities in appearance with the Headcrab (with the Mortar Synth, it is one of the only Synths to have "Synth" in its name). It has incomplete animations, model errors, both high and low quality textures and missing collision files, among others. This is likely why it is only glimpsed and not confronted during the game.
Designed by Dhabih Eng, it is one of the earliest created Synths (with the Mortar Synth), when they bore more red and yellow and did not feature any Combine logo (see also the original Dropship). Ted Backman subsequently defined the current design, with more blue.
The Crab Synth is an NPC in the Half-Life 2 Beta; however, it does not work correctly. According to its source code, it was designed to charge at the player. It is impervious to bullets and can only be killed by causing an explosion underneath it, but it lacks a death animation and ragdoll parameters.