Squats are a race that were playable in first edition of Warhammer 40,000, but have since been removed. They were analogous to dwarves, being short abhumans from high-gravity worlds. They have since appeared on and off in canon, with the revised version of the novel Draco by Ian Watson claiming that their worlds were destroyed by the Tyranids. However, some squats are still alive, such as mercenaries featured in the board game Necromunda.
The canonicity of the Squats is played around with in If The Emperor Had a Text-To-Speech Device. Their ships were seen being eaten by Tyranids in the BEHEMOTH Trilogy, and they now only appear in the Retconnian alongside Malal and Horus' soul. Magnus and the Emperor later confirmed that the squats were eaten by Tyranids while covering the novel Draco. The Emperor off-handedly remarked that he told Roboute Guilliman that he was not allowed to reinvent Squats. 
Dwarves are nearly always referred to as Squats, even if they come from the Warhammer Fantasy universe. One such dwarf who appears in TTS is Urist Boatmurdered, who transfers his hatred of elves onto the equivalent Aeldari (in particular the Drukhari).