Gaper and Mulligan variants are the only enemies with "normal" legs (Gaper is almost identical to Isaac,and Isaac is a human)
So,guess the first form of the Mulligan is Gaper,he was normal,but flies start trying to consume his body,and he turned into Mulligan,he is scared about this,thats why he dont attacks you
That just a theory...
A GAME THEORY!
Thanks for reading!