A psychological assessment of Cabrera that showed he was fit to stand trial resulted in his guilty plea.
The other man arrested for horse butchery, Luis Cordero, 18, is still considering the plea deal, which is open until mid-January. If he does not accept the deal, his case will go to trial.
The animals were butchered for their meat to fuel what police believe is a lucrative black-market trade.
The string of killings outraged local horse owners.
Cabrera admitted charges of armed burglary, animal cruelty, killing a registered breed horse and breaking a fence in connection with the incidents, both committed with the intention of selling the horse's meat.