The Omilteme cottontail (Sylvilagus insonus), or Omiltemi cottontail (Spanish: conejo de Omiltemi), is a species of cottontail rabbit in the family Leporidae found only in the Mexican state of Guerrero in the Sierra Madre del Sur mountain range. It is a nocturnal, large rabbit, rufous to black in fur color, with long ears and a short tail. This cottontail is restricted to cloud forests at elevations of up to 11,499 feet (3,505 meters).

First identified in 1904 by Edward William Nelson in Omilteme, Guerrero, Mexico under the scientific name Lepus insonus, the Omilteme cottontail was soon after reclassified as a member of the genus Sylvilagus, the cottontail rabbits. It is closely related to the Mexican cottontail (S. cunicularius) and the desert cottontail (S. audubonii); the former species overlaps in distribution with the Omilteme cottontail.

The Omilteme cottontail is considered one of the most endangered mammal species in the world, and is only known from a few specimens. Once listed by the International Union for Conservation of Nature and Mexican authorities as critically endangered, it is now considered a data deficient species. From the early 1900s up until the 1990s, no confirmed sightings of the cottontail were recorded, but two individuals were discovered in 1998, and more were found through the 2020s using camera traps. It is threatened by poaching and habitat destruction, and much about its natural history is unknown.

The Omilteme cottontail was first described by Edward William Nelson in 1904 as a new species of Lepus, "Lepus insonus". The description was made based on two specimens, which were collected with the assistance of Edward Alphonso Goldman. The type of this species was a female specimen collected from Omilteme, Guerrero, Mexico, stored at the National Museum of Natural History. It was described as "[a] dark, coarse-haired species with small short tail" and noted as "obviously belonging in the same group as [Lepus] gabbi and L. truei", though it had ears that were twice as large as either species. Both L. gabbi and L. truei are now known as subspecies of the Central American tapeti (Sylvilagus gabbi), a cottontail rabbit of wide distribution and unstable taxonomy. In June 1904, the month after Nelson published his description of Lepus insonus, Marcus Ward Lyon Jr. published his work Classification of the Hares and their Allies that clarified the genera of hares and rabbits. Nelson referred to this work in his 1909 publication The Rabbits of North America, where he placed the Omilteme cottontail in the genus Sylvilagus.

There is no fossil evidence of Sylvilagus insonus, and it is only known from its type locality in the province of Guerrero, Mexico. Phylogenetic analysis of the relationships between S. insonus and other species in the genus Sylvilagus yielded evidence that it is closely related to both the Mexican cottontail (S. cunicularius) and the desert cottontail (S. audubonii). The former species is sympatric with S. insonus. Prior studies indicated a relationship with the common tapeti (S. brasiliensis), and it has been variously placed in the subgenera Tapeti and Sylvilagus, but morphological studies find that the common tapeti is more closely related to Dice's cottontail (S. dicei) than it is to S. insonus.

The Omilteme cottontail is a large rabbit (head and body length from 15.7 to 17.3 in (398 to 440 mm)) with long ears (2.4–3.0 in (60–76 mm)), hind feet of medium length (3.5–4.1 in (89–104 mm)) and a short tail (1.6–1.8 in (40–45 mm)). Around the nose and orbital (eye) area, its fur coat is a dull gray. The external, convex surface of the ears is a dark brown-black color; the black is concentrated along the border and tips of the ears. The rabbit's back is rufous (a red-brown color) mixed with black, while the sides are gray-black in color. The medium-sized hind feet are white on the dorsal side and the soles are a dark brown. The dorsal side of the tail is reddish-black while the ventral side is darkly buff or yellowish.

The anatomy of Sylvilagus insonus has been described with particular focus on the skull. The skull is large, approximately 3.1 in (78 mm) in length and 1.3 in (32 mm) in depth, with a large palate and a wide braincase (back and upper part of the skull; neurocranium). The supraorbital process (projecting bone structure above the eyes, or brow ridge) is flat, attached to the braincase and has two extensions (anterior and posterior). The anterior extension of the supraorbital process is attached to the skull, while the posterior extension is slender and can be free of the brain case or attached with a slit in between the process and the braincase. The supraoccipital shield (bony structure above the occiput) is square shaped. The tympanic bullae (bone structures that enclose the middle and inner ear, synonymous with but measured differently from the auditory bullae) are small and less than 0.48 in (12.3 mm) in length. They have medium-sized auditory bullae with a length of less than 0.38 in (9.6 mm). The width of the basioccipital is narrow: less than 0.35 in (9 mm), but broad across the carotid canals. The width of the infraorbital canals is very narrow, being less than 0.72 in (18.3 mm). The width across the nasal structures is very narrow, and their length is less than 1.26 in (32.1 mm).

The mouth consists of a mandible whose height is less than 1.43 in (36.3 mm), with a mandible ramus depth of less than 0.44 in (11.3 mm). The incisive foramen and the diastema are short. The premaxillaries have extensions on the dorsal side. Large maxillary and mandibular tooth rows are present. Like other leporids, the Omilteme cottontail is heterodont with a total of 28 teeth; it has incisors, premolars and molars, lacks canines, and its dental formula is 2.0.3.31.0.2.3. The length of the first upper incisor is generally less than 0.30 in (7.5 mm).