Crackpot theory is that Robb warged into Grey Wind, then into Raynald (his second life as explained during Varymyr Sixskins prologue chapter in DwD) after Grey Wind started getting attacked.
In Jaime's chapter, in FFC, where Edmure finally surrenders Riverrun he asked about what happened to Raynald. One of the Frey's tells Jamie that Raynald was really meek during the Red Wedding ("he gave up his sword without a fight") but as soon as they went after Grey Wing Raynald flipped out and started killing Frey's left and right. Then, after getting shot in the stomach and the shoulder he slipped into the Trident and no one has seen him since.
This post was edited on 9/24 at 1:54 pm