Daemonic Speed
The model has mutations such as long limbs or four legs which allow it to move as Cavalry (see main rulebook, page 93).
The model does not have to move at full speed and may always choose to move as infantry in any Movement or Assault phase. If it disembarks from a vehicle in its Movement phase then it can only move as infantry in the subsequent Assault phase.
Errata — Chapter Approved: Codex: Chaos Space Marines Q&A (v3.0): Daemonic Speed grants a 3D6" Pursuit/Fall Back move, the same as Daemonic Flight and a Daemonic Steed. Daemonic Speed cannot be combined with Daemonic Flight — a model has only one special movement mode, however it's obtained. Followers such as Chaos Hounds or a Chaos Spawn move and assault at their owner's speed only as infantry — if the owner has Daemonic Speed, Flight, or a Steed, the followers cannot keep up and the owner is not slowed down to match them either (the exception is Blood Rage, where it's reasonable to let a Spawn keep pace with its raging owner).