Not all hybrid animals are sterile
In equines, it seems that some female mules can be fertile, even though horses and donkeys have different numbers of chromosomes. In hamsters, however, both Campbell’s and Winter Whites have 28 chromosomes so hybrid offspring will also have 28 (Chinese hamsters have 22 chromosomes and Roborovskis have 34 and these species can't hybridise with each other or the Campbell's and winter whites).