Maybe you could make their reproduction rate lower then the other monsters. And/or they don't show up at the Market as often
Make them cost more to buy. They are more valuable to you because they fill both Male and Female slots in one, and if they don't show up to in the Market as often then they are Rare monsters Justifying Increased price as well
Having a lower Reproduction rate would explain why they aren't in the Market as often
I'm glad I could help. Adding in lore like that could explain why the game works a certain way. You Have the "Monster Ranchin for Dummies" In the Barn. Maybe you could put next to it a "Bestiary". Have a Pic of the monster with name, What role it plays in breeding, and a paragraph about it. Right now each monster has only one gender. So the roles would be Male, Female. If you come out with Male and Female versions of the monsters you can remove that piece. If you do make Male and Female of each monsters maybe cross breeding is harder. And Hybrids are an idea as well
