There are certain paranormal beings I've come across that are not manifestations at all. In fact, these creatures seemed to be flesh and bone, having walked through some portal leading from their parallel plane of existence to ours, veritable demons on earth. This is usually the result of some sort of summoning, and as such, these beasts are extremely hard to get rid of. What little damage I have seen dealt to one seemed to simply anger the thing, altering its demeanor to an even more terrifying state.
Similar to the "Terror Dog ", we encountered during the Gozer incident, I've seen physical beings of this nature. As Tobin stated, they became much more aggressive when attacked, but after repeated bombardment with our equipment, they were eventually subdued.
These little devils (and I mean that literally, these are demons, not ghosts!) can be pretty nippy, and usually come in packs. Bowl them over with the Shock Blast or Boson Darts.