Simulaquins, by their nature, are 'blank' beings that lack apparent 'Individuality'. They can still retain thoughts and interests of their own, as well as even some form of agency, but it is very easy for them to assume roles that are given to them.

This can be especially true for Simulaquins that have ended up in much more 'object-based' forms. Before any sort of modification, they just exist as they are. However, when modified through a coat of paint and additional features, they are capable of actually BECOMING what they have been made to be. A blank slate being remade into something else.

As such, this would be why 'Cupcake' was able to go from a simple blank rocking horse template to an actual magical unicorn. It was because that is the role and 'intent' bestowed.