EntityMobCow1, EntityMobCow2, etc type variables should be replaced with an easier to read method. I propose: every entity gets a handler which stores all the details about it (EntityMobSpawnProb shouldn't be a huge list like it is for instance, instead directly associated with its handler) and those handlers are in an array like so:

EntityMob["Cow"][0] = vanilla cow
EntityMob["Cow][1] = start of list of modded cows