Koko the horse appeared to be a typical mare, but when she began acting overly aggressive and trying to mount her female counterparts, her owner became suspicious.

After a barrage of tests at the University of Guelph’s Ontario Veterinary College, researchers discovered that Koko was in fact a male horse that appeared female.

The scientists tested Koko’s relatives and found her sister Sequoia and her cousin Pandora were also intersex.

Researchers Allan King, BSc, MSc, PhD, Tracey Chenier, DVM, DVSc, and Daniel Villagomez are the first to identify a family of horses with a rare genetic abnormality called pseudo-hermaphroditism, which causes genetically male horses to appear female on the outside