It's weird, right?
I changed here, in the knight_familiar.lua.
This is the original code, my change was in the looktype, i simply changed the number 991 to 21 (or any other) and it showed the invis sprite.
local mType = Game.createMonsterType("Knight familiar")
local monster = {}
monster.description = "a knight familiar"
monster.experience = 0
monster.outfit = {
--lookType = 991,
lookHead = 0,
lookBody = 0,
lookLegs = 0,
lookFeet = 0,
lookAddons = 0,
lookMount = 0
}