The Phobovores were small, grey humanoids with long noses. They came from another dimension and in their own right were dull rather than scary. They fed on fear — fear generated in others. Only with that fear could they exist in our dimension.
They used a "Ghost Train" ride at a fair to scare people whom they then kidnapped for a few days by hooking them up to a machine that created nightmares while they fed. The hosts were released unharmed a few days later, with no memory of what had occurred. Rory Williams was not a good choice of fear provider because his faith in the Eleventh Doctor and Amy Pond helped him overcome his fears and the Phobovores were left weakened and forced to return to their own dimension. (COMIC: Ghost Train)