Merycochoerus (Pronomotherium) californicus was named by Merriam (1919). Its type specimen is UC 21351, a partial skull (Fragmentary skull with P3-P4 alv. and M1-M3 (M2-M3 br.)), and it is a 3D body fossil. Its type locality is Ricardo Ampitheater, which is in a Clarendonian terrestrial horizon in the Dove Spring Formation of California.

It was recombined as Metoreodon californicus by Matthew (1924), Thorpe (1937); it was synonymized subjectively with Merychyus major by Lander (1998); it was recombined as Ustatochoerus californicus by Schultz and Falkenbach (1941), Stevens and Stevens (1989), Stevens and Stevens (2007), Stevens et al. (2023).