the giant hippo-like traditional rhino, Metamynodon planifroms (Scott and Osborn 1867; Early Eocene; 4m long), nests with Eotitanops, the basal brontothere, though not far from Ceratotherium, the white rhino.
Amynodon advenus (Marsh 1877; 1m in length; Oligocene-Eocene, 40-23 mya) was originally considered an aquatic rhino. Here it nests with Mesohippus. The long neck and other traits are more horse-like than rhino-like. Manual digit 5 was retained. The skull was deeper as in basal forms like Hyracotherium.
nest horses separate from tapirs + rhinos. The LRT nests horses with rhinos both derived from a sister to Hyracotherium and a sister to tapirs + chalicotheres. Traditional cladograms also avoid mixing brontotheres, horses and rhinos, like we do here.
Marsh OC 1877. Notice of some new vertebrate fossils. American Journal of Arts and Sciences 14:249-256
Scott WB and Osborn HF 1887. Preliminary account of the fossil mammals from the White River formation contained in the Museum of Comparative Zoology. Bulletin of the Museum of Comparative Zoölogy at Harvard College 13(5):151-171.