Black Beauty experiences a number of health issues throughout the story, many of which are related to his work. One such example comes when Beauty dashes off at break-neck speed to find a doctor to come and tend to the Squire's wife, who's seriously ill. Although Beauty helps save the woman's life, he unfortunately falls sick himself, exhausted as he is by his endless comings and goings. His condition is made worse by the unintentional neglect of his new groom, the gentle but inept Joe Green.
Neglect is also much in evidence when Beauty's temporary caretaker, a drunk by the name of Reuben Smith, fails to maintain his horseshoes properly. This leads to Beauty's legs collapsing part-way through a journey. In turn, the unfortunate Smith is thrown from his horse and killed instantly. For his part, Beauty is required to submit to a very painful medical procedure that leaves his legs in a permanently scarred condition.
Beauty's health deteriorates sharply later on in the story when he starts work as a cab-horse. He's chronically over-worked and exploited, driven way too hard as this is the only way that the cab-drivers can cover the rent that they must pay for hiring the horses. Beauty's health is completely destroyed during this time, but thankfully he's soon able to recover after being sold to the appropriately-named Farmer Thoroughgood, who treats Beauty with kindness before eventually selling him on.