Spider Butt Disease

AKA "El Trasero Noctaraña". An exceedlingly rare medical condition in which the afflicted's gluteus maximus transforms into a giant spider at night.

Symptoms

 * The primary symptom is the nightly transformation of the posterior into a giant spider. The effect typically persists from midnight until sunrise.

Cure
The cure for this disease is the injection of a serum composed primarily of the Tusk of the Monotooth Jackal. This is fatally poisonous to anyone not suffering from this condition. The treatment is effective within minutes; the patient's buttocks revert to their normal form, and the spider is reduced to a small purple puddle.