"External" predefined values

I was wondering whether I could set a “Predefined values” constraint for a string field, by picking them from somewhere “outside” of the Content-type.
For instance, from some content or a separate list.
What I’m thinking if is: let’s say I want to record a collection of dogs, each one with a name and a breed. I would like to avoid listing the breeds in the Content-type definition, specially cause I would also like to use the same for restricting the applicable breeds for a collection of collars. When one added a breed, the lists would be automatically updated.

I anticipate a possible solution: add the breed as a link to a new Breed content-type.

Yes, it would work, but that would make the API responses terribly longer, for just a string.

Is there the possibility to avoid doing it, or at least return only the breed code?
Thanks in advance!