We studied factors affecting separation of feral horse foals from their mothers and foal mortality, including variables such as group changes by the mother and behavior of the foal. We include data on feral horse foals that died or were separated, and their siblings. We conducted this study between 2017 and 2020 at two Herd Management Areas (HMAs) in western Utah, USA: Conger and Frisco. We conducted demographic observations year round at both HMAs to record group composition and foaling rate. We additionally recorded behavioral observations for a subset of foals at Conger HMA annually between March and September. This dataset includes 2 .csv files: Data_foals_dead_separated_siblings.csv - These data contain 52 unique foals across Conger and Frisco HMAs with associated data on mother and group parameters. Behavior_foals_dead_separated_sibs.csv - These data contain behavioral observations recorded on 15 foals just in Conger HMA.