This is the NPC Butcher Slime... THIS AINT CUSTOM, it is present in texture files and, in this quote from the mod, "Other than the above slimes, there are also slime villagers (only spawning when you make slime village buildings) and a few other enemies found only in certain structures.", so this is a Slime From the "slime Village".
~Aneliomo Link to the mod: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1282881-1-2-5-forge-slime-carnage-v1-1-1-500-downloads
Template by JD, Slime template credits to him... Photos of the slime by me