Finally the PBS Eons narrator spilled the beans
on why mammal workers traditionally considered Paraceratherium (Fig. 1) to be a giant hornless rhino. It was (as I suspected, but never learned from emails to rhino experts) the shape of the molar cusps. That issue is addressed below.
The following are my two comments yesterday
in the YouTube comments section:
Not a rhino! A giant three-toed horse. Molars are convergent between closely related horses and rhinos. Details here: https://pterosaurheresies.wordpress.com/2016/12/30/maybe-paraceratherium-is-really-a-giant-horse/ and here: https://pterosaurheresies.wordpress.com/2017/01/03/more-taxa-for-the-paraceratheriumgiant-horse-hypothesis/ and here: https://pterosaurheresies.wordpress.com/2018/11/11/aceratherium-vs-paraceratherium/
PS. Hyrachyus IS basal to rhinos. Pappaceras nests between two 3-toed horses (Mesohippus and Miohippus) and Juxia is the size and shape of a horse because it nests between Equus (the modern horse) and Paraceratherium. If you exclude horses from the analysis, sure Paraceratherium will nest with rhinos. When horses are added to the analysis Paraceratherium and Juxia nest with Equus. Don’t trust just one character like molar shape. Look at all the traits from nose to tail and then do your analysis. More details here: https://pterosaurheresies.wordpress.com/2017/04/27/indricotheres-horse-like-rhinos-or-rhino-like-horses/