Two quadrigeminal arachnoid cysts with different pathogenesis are described in 2 different species. A 10-week-old male Persian kitten with a progressively decreasing level of consciousness died spontaneously. At necropsy, mild internal hydrocephalus, caudal cerebellar coning, and cerebellar herniation through the foramen magnum were associated with a congenital quadrigeminal arachnoid cyst compressing the rostral cerebellum and shifting the entire cerebellum caudally. In contrast, a possibly acquired quadrigeminal cyst was observed in a 2-year-old male neutered Yorkshire Terrier in association with necrotizing encephalitis. Quadrigeminal arachnoid cysts have been rarely reported in dogs and humans.