[@Archangel89] - Generally speaking gods can influence most things but to a much lesser degree than gods who belong to the domain being influenced. For example if Dirka, goddess of magic, tried to turn steel swords into snakes but Ferron didn't want her to Ferron's power would win out since swords are made of steel and not magic.

So in short it's entirely possible for Iarus to make snake people. He'll just have less sway over them directly than some other gods may.

Edit: It also may cost a great deal more effort to create them than it would for gods with relevant domains like animal gods.