From the owlbear to the manticore, wizards have loved mashing up critters from the very beginning. Here are some rules.
Note: Crossbreeding is a total waste of money, as few will be interesting in purchasing your vulturedogs. However, MUs which successfully breed a new species (producing at least one mating pair) will earn one XP for every GP spent on the project.
Basics of Birthing a Cowfrog
Crossbreeding is a remarkably difficult process to pull off. While there are a handful of natural crossbreeds (mules, ligers, centaurs, etc.), more unusual pairings will require both Animal Magnetism and the Ritual of Twaining (see below). Once both of these spells are cast, there is a base chance of the pair mating equal to the average of their individual fertility rates + 5% per level of the MU. This is modified by the following:
Size: -10% by size category
Limbs: -15% per pair different
Type: -30% if different clades (fish, fowl, mammal, reptile, etc.), -50% if wildly divergent (e.g. a dog and a jelly)
Example: Philip the Bloody, a 7th level MU, intends to breed a crow and a rat. The LL determines that crows breed 40% of the time, while rats breed 80% of the time. The average of these two rates is 60%, +35% for Philip's level, giving the crowrat a 95% base chance of breeding true. However, because the crow and the rat are of different clades, the chance is reduced by 30%, and the difference in limbs reduces the chance by an additional 15%, giving the crow and the rat a 50% chance of producing a horrible little monster.
Is it More Cow or More Frog?
If successfully bred, the LL will then determine which characteristics are gained from the parents. The LL may pick and choose according to his whim, or give a base 50% chance that each trait is inherited. Traits of each species are to be determined by the LL, but will cover things such as the number of limbs, tails, fur, feathers, prehensile fingers, etc.
Ritual of Twaining
Casting Ritual of Twaining allows for the successful mating of normally incompatible species, overcoming both mismatched genitals and genetic infeasibility. This ritual requires three hours of preparation to cast. Further details would be in bad taste.