If you are successfully setting the synonym like Keebo suggested:

function male_fire_mutant
{syn_male_fire_mutant = ME;
}

... where this function is the action you assigned when you created the model (with your create statement), then this should do it:

syn_male_fire_mutant.invisible = ON;

I just tried this last night in my game and turning visible off didn't seem to do it.

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