Scholars had long likened themselves to horses. Mistreated horses are still noble animals, like the noble but maligned scholars. Thin horses could represent the scholar who suffers poverty rather than work for a corrupt government, but could also represent the scholar-official who is so devoted to the welfare of the people that he grows poor in office.
Ren's painting is actually part of a larger composition, with a fat horse and this thin horse. Ren's inscription says that the fat horse represents the prosperous official who uses his position to enrich himself, while the thin horse is the self-sacrificing official who grows thinner from serving in office.