The Horus Heresy was fought between the Space Marines loyal to Horus and those loyal to the Emperor. The traitor legions, of course, being equipped just as any other space Marine legions, had Titans. Soon, you'll be able to bring them to your tabletops with Adeptus Titanicus: Traitor Legios, a new book coming for the game.
From the article:
Worshippers of the Chaos pantheon, your heretical prayers are about to be answered in the form of a new tome dedicated to the annihilation of the Emperor’s pitiful servants.
The Traitor Legios book is the definitive guide for those who pledge their god-machines to the Warmaster. Strap your arms and legs (or mutated bundle of tentacles) into your restraint harness, and get jacked into the Manifold – things are about to get gross.