Gell guard

Gell guards were blob-like creatures created by Omega, trapped in a black hole domain made of anti-matter.

Biology
They had the ability to travel from Omega's anti-matter universe into that of matter. They were unaffected by gunfire and explosions and could use their claws as weapons to create explosions of their own. (TV: The Three Doctors)

History
Under Omega's command they attacked UNIT HQ. They killed a number of soldiers, captured the Third Doctor and his friends, and took them to Omega. (TV: The Three Doctors)

Other references
The Gell guards were one of the species referenced in Theoretical Monsters: A Credibility Test. (PROSE: Interference - Book One)

Behind the scenes

 * The name "Gell guard" is never used on-screen. It first appears in the Doctor Who universe in Interference - Book One.