LHAASO-KM2A is a sub-array of the Large High Altitude Air Shower Observatory (LHAASO) with an area of kilometer squared scale, consisting of 5195 electromagnetic detectors and 1171 muon detectors. Horizontal Air Showers (HAS) are a fundamental tool to detect penetrating particles like neutrinos and to study hadronic interactions. HAS detected at the ground are mainly constituted by secondary muons. In this contribution first observations of HAS with half array of LHAASO-KM2A are reported. We show that the zenith angle distribution of extensive air showers (EAS) and the transition from electromagnetic-dominated showers to muon-dominated ones above a zenith angle of 60 • .