Pleco attacking other fish?

What kind of pleco and what else is in the tank? What are you feeding it? How big is it? Some plecos like more meat than others and it is not impossible for a starving fish to do just about anything it possibly could. to survive. Still, I would like to hear more before I blamed a well-fed pleco.

Gouramis can be very territorial, the gourami could have been the one that bit your catfish. There are also diseases / conditions that cause ulcerations and holes in fish that can look like bites.

If the other catfish and the neon died somehow it's entirely plausible that your plec had a munch on them after they died.
I've heard of starving plecos going after fish, but I think they usually try to suck on them and eat their slime coating, I'm not sure a plec is even capable of 'biting' in the traditional sense since their mouths are disc shaped.

Are you target feeding your plec? If not, you could have a starving fish on your hands who will do anything to eat.

If you didn't see anything there's not telling what could have happened. As for the shape of the fish when you saw them, when a fish dies usually the eyes are the first to go then the reast of the head. After these goodies are gone they go for the bellie. My point is most all fish will peck off of a dead fish. If you're basing your assumptions on this it's prob not a very good thing to go off of.

As for the bite on your catfish. Have you ever looked at a pleco on the side of the aquarium? They kinda have a tongue like thing that licks their food I guess you could say. If the pleco were to 'inflict' a noticable 'bite' he'd have to be sucking on the catfish for a pretty long time before you noticed it.

What else is in your tank is the question. Any noticable stress showing from any fish? As stated it could also be an infection of some sort.