I'm playing 1.12.2 version of MC Eternal with Rats Version 3.2.14-1.12.2.jar.

And I noticed in my SSP world every rats that spawned is not a regular rats but instead it's spawned as like this

And there are a lots of them if I do /forge entity list, it will spawned around 70 - 90 rats. I tried using /cofh kill rats and it will just spawned again like this.

This thing has breedable cooldown, and if I wait that timer it will turn into regular rat. And if I get near this sometimes it will jumps on the same spot for a couple of times.

Is this a mod conflict issue?

okay it is a mod conflict issue the mod is RandomTweak, in this mod there is a feature where it tries to randomize animal's age.