I forgot that the bat was considered one. I think for the bat, because it was able to terrorize the other ghosts it was able to generate the fear it needed to gather and feed on to be stronger. I mean, if a bat could eat you I would be scared too.

But yeah, I believe that negative emotions can come from animals, but not like a normal “survival” type of death, like that of flies. To me, the negative emotions have to result from a curse. From something like all of the pain and suffering and torment that a creature went through before it died. A good example of what could be an animal “grunge” is something like massive animal experimentation, to the point where the spirit decides to get revenge. 

A great example would come from an anime called “The Ancient Magus’s Bride” where the focus is on cats. Won’t give too much away there, but it is beautiful animation and would recommend for people to check out.