I think minor mutations would be cool to be able to buy for existing characters. The ones Thoxix suggested sound good to me. But also maybe slowly loosing hair/growing more hair, or eyes slowly shifting to a new color (natural or unnatural, it'd even still be cool if it was restricted to a shift from one natural color to another i.e. brown to blue or vice versa.)
I'm not sure if I like this idea. I like the aspect of people working to get hp to buy things so that they can make mutated characters. I mean, obviously it's the same concept if you plan to mutate a character in the future, but I think people would make a lot more characters if they were allowed to buy mutations for them later. And then there would be a plethora of characters that would hardly get played and such.
Buuut, if this was to come into play, I say that mutations for an existing character should be twice as expensive than if you were to buy it for an unmade character. And like Thoxic said, just minor ones.
Well, the case can be the other way around too, people might create less characters if they could get the mutations they want for already existing ones, since I know that several people create a new character only to get certain mutations, because thats the only way to get them except breeding. So for every idea you come up that has a mutation, you have to create another character, and then another for the next idea and so on. But yeah, as I said, minor things would be cool, since a mutation might lie dormant inside the horse : O And maybe a limited amount of mutations on such cases? So that they can't mutate into looking completely different, but only get a few "extras" so to speak, like horns, unnatural eyecolor och split hooves, or maybe they go bald/extra hairy? Or maybe their tail start to grow into a lions tail or whatever, you get the point xD But maybe they can't change coat color, grow wings and such things. And it has to be a very gradual thing, at least in my opinion, so it takes several seasons before the mutations is done.
I think gradual and minor would be the key for mutations for existing characters. And I don't think being able to buy mutations for existing characters would lead to a surplus of characters like Thoxic said. Fewer people would be making a new character just so they can have a character with just a single mutation that isn't drastic.
Though I also do think a limit for how many post-creation mutations you can have per character is a good idea. Like they can only have like 2 or 3 "dormant" mutations.