I appear to be awful at describing the question so I'll try and describe the problem.
I want to add a random amount of heads to my creatures but I want to be able to determine several things. a) The minimum number of possible heads b) The maximum number of possible heads c) The probability of the number being high/low within the above values.
So i could add heads like so: addHeads(5, 10, 0.5); // should produce creatures with "around" 7.5 heads but they could have anywhere from 5 to 10.
So random number generation isn't the problem, but controlling and actually using them in a game is. :D