Cancer (disease)

Cancer is a disease that's been indirectly described as being something that spreads throughout one body until its to late. Zecora named the curse from Abyssinia "Ethical Cancer" after the disease, given how it affects ones personality similarly to how cancer affects ones body.

A pony with no fur, mane or coat was seen in "Read It and Weep", leading many fans to believe he may have had a form of cancer.