Level 11 Monster Plant

Hi, I don't really like to post on forums too much, rather I enjoy just reading them. However, this was driving me nuts. A while back I saw a level 11 Monster Plant seed in a coinshop, and bought it. I have never seen a level 11 monster pet, and thought they only go up to level 10. I kept it for a while thinking someone may pay a good amount of coins for this, but nobody seemed interested so I planted it. I just breeded it with a level 10, and received a level 10 seed. I'd like to breed it again, but the rebreeding potions only go up to level 10. Would that work, or no? Also, do these level 11s more common than I think? Anybody that I have ever asked says they have never heard of them.