Crab Synth

The Crab Synth is never encountered in combat and is only glimpsed towards the end of Half-Life 2 during Gordon Freeman's second pod ride in the chapter Our Benefactors. The Crab Synth bears a slight resemblance to the Headcrab.

Capabilities
From the Crab Synth's appearance, it looks as though these types of synths can use powerful claws on the front, and can fire at enemies using the pulse turrets visible on the front. The Crab Synth looks to be something that would be used by the Combine forces as a light support synth.

Source SDK
In the Source Development Kit Model Viewer, it can be seen that the Crab Synth uses several melee attacks and can charge at enemies.

Trivia

 * Crab Synths can be spawned in Garry's Mod without crashing the game, however, it will remain idle and you can walk through it.