Prevalence of twin foaling and blood chimaerism in purebred Spanish horses.

•This is the first comprehensive study assessing the prevalence of twin foaling and blood chimaerism in horses.•Foaling data were analysed from more than 21,000 purebred Spanish (Pura Raza Español) horses born in 2015–2016.•Chimaerism was detected using a novel and validated molecular technology based on short tandem repeat markers.•Twenty-three live twins and five chimeric individuals were identified, establishing prevalence data in this breed.