Ilyodon is a genus of livebearing fish in family Goodeidae - the splitfins - restricted to the freshwater streams, canals, and lakes of the Mesa Central highlands of Mexico. Like other goodeids, Ilyodon fertilizes internally through a modified anal fin in the male, and the embryos develop inside the mother for weeks before birth, drawing nutrients not through a placenta but through ribbon-like trophotaeniae that trail from the young fish's vent - the reproductive mode that sets the family apart from live-bearing poeciliids such as guppies and mollies. The atlas covers 4 species, all 2–3.5 in, with the blunt, close-set teeth typical of algae-grazing goodeids.
The group draws attention mostly from specialist livebearer and goodeid keepers rather than the general trade - species such as the Balsas splitfin (I. whitei) and Goldbreast splitfin (I. furcidens) are grazing, temperate-tolerant fish suited to cooler, well-oxygenated setups that echo highland Mexican streams. Wild goodeids as a group are in serious trouble: introduced tilapia and poeciliids, plus heavy water extraction, have hit the Mesa Central hard, and one Ilyodon species carries CARES status in the atlas as a result. Of the 4 species, 3 are assessed Least Concern and 1 is Not Evaluated, so the tally alone understates how much conservation concern surrounds the wider family.