OMG i totally forgot it, quite obvious actually, the data/1 function on pets was like this:
%{
id: pet.id,
name: pet.name,
bio: pet.bio,
photos: pet.photos,
age: pet.age,
gender: pet.gender,
breed: pet.breed,
species: pet.species,
vaccinated: pet.vaccinated,
dewormed: pet.dewormed,
neutered: pet.neutered,
disability: pet.disability,
pedigree: pet.pedigree,
size: pet.size,
weight: pet.weight
}
Removing this breed key and pet.breed value solves the error, and the reasons why the error was not happening with the :users association is because on data is not trying to render users, thanks for this comment bro 
