The term "enslavement" is typically used to describe the forced and unpaid labor of human beings. While it is important to recognize that animals, such as horses, can be mistreated or exploited by humans, they do not have the same capacity for freedom and autonomy as humans. Therefore, comparing their treatment to slavery may not be entirely accurate or productive. A humanzee would be 50% human, so therefore it will be morally wrong to have humanzee slaves.
That being said, there are certainly ethical concerns surrounding the use of animals for human purposes, including work, entertainment, and food production. Many people believe that animals deserve certain moral considerations based on their ability to feel pain and suffer, among other factors.
In the case of horses specifically, some argue that using them for work or riding can be beneficial for both the animal and the human involved, providing exercise, companionship, and a sense of purpose. However, it is crucial that horses are treated with respect and care, given adequate rest, nutrition, and medical attention, and trained in a humane and positive manner. Unfortunately, this is not always the case, and many horses end up suffering from neglect, abuse, or overwork.