The Mobs folder includes 4 varients of most mobs. 
This is meant for Farn's Random Mob Texture Mod: 
https://github.com/FarnGitHub/Farn-s-Random-Mob/releases/tag/1.3.1

You can choose to rename one of the varients if you like it, 
just delete the png that does not have a number and rename - 
one of the other png's to not have a number!


For the files named 
"Custom_[BLANK].png"
These require a version of Optifine or MCPatcher mods
for them to be used ingame.