Mutations are special traits that are normally not present in Pacapillars. Each mutation has their own rules, which are described in detail in their encyclopedia page.
There are two categories of mutations: Mutation and Mutation+ (read as Mutation Plus). Mutation+ traits are different, more rare, and often (but not always) are variations of normal Mutation traits.
These traits are unlocked using an Olera Berry for Mutation, and a Ripe Olera for Mutation+. Each berry gives access to one trait, so if you want to add two mutations, for example, you would need two Olera berries. There are also seasonal items for Mutation+ traits, which give a specific trait depending on the item; you'll be able to learn more about berries and traits in the MYO Guide!
Mutation traits are Pincers, Multi-Texture, Shedding, Single Tail, Slug Body, Split, Chameleon, Paws, Spider Eyes, No Eyes, Body Eyes, Cyclops, and Animation , you can read their specifications in this page.
Mutation+ traits are Ultra Tail, Multi-Texture (Plus), Octo Body, Object Body, Object Head, Overlay, Split (Plus), Chameleon (Ultra), Fins, and Animation+ (Segmentation), you can read their specifications in this page.