Mutations can be very common in SciFi books but doing them right may be difficult. No one wants to come across as cheesy or cliche when it comes to body mutation so here are some tips on how to successfully mutate your character (wow that sounds so messed up I can't even)

1. Once you mutate your character they can't go back (unless they get un-mutated). From now on, you have to remember your character's mutations and what those mutations may do to their body.
2. You have to think about how the mutations will affect your character. If they suddenly have horse legs, walking may be slightly different.
3. You have to have a reason for them to be mutated. It can be a really obscure reason but it still has to be a reason.
4. You have to figure out how people will react to the mutation of your character. Will they be shocked? Or sad? Or scared?
5. You have to figure out what your character does with their new mutation. Does it help them escape from somewhere? Is it useless?

Basically, if your character has a mutation you need to think really carefully about the way you describe your character.