Colorado Parks and Wildlife: Scary-looking rabbits were hopping around Fort Collins. These weren’t your standard cute, fluffy bunnies; they had horn-like growths protruding from their faces and bodies.

Viral photos have inspired a fluffle of unflattering nicknames, including "Frankenstein bunnies," "demon rabbits" and "zombie rabbits."

Colorado Parks and Wildlife officials have identified the cause as shope papillomavirus, a rabbit-only virus with no cure that produces wart-like tumours known as papillomas, particularly around the face and head of cottontail rabbits, 9NEWS reports.