It's the same as the three we already have, except brown and a bit smaller.What that rhino did to you.
The Indian rhino is from the base game, and I have no problems with it, but from the start it was clear that they were eventually going to shoehorn one of the African rhinos in a later DLC...
And so we got the white rhino. I would have liked this animal to be in base game instead of the Indian one, but oh well.
...and then Arid Pack rolled around. Now, I loved most of the animal selection in Arid (especially the gazelle, addax, wild ass and dromedary), but the presence of our third rhino made me seriously consider skipping the pack. Especially considering that, while I can see a few minor differences between the white and Indian rhinos (which also have different ranges in different continents), I can't find any significant difference between white and black rhinos. They look virtually the same, they're from the same area, the only real change is the colour and the addition of the desert tag. Some people will point to its different diet and ecology, but how does that reflect in game? It just has two extra enrichment items that the white rhino can't use (but the Indian can).
The Sumatran rhino would basically be a carbon copy of the Indian one, except with a few brown markings here and there. It's basically a cosmetic variant of an animal we already have, an animal we already got THREE separate times. In addition to that, it's not even found in captivity anywhere so really it's even more pointless.
It's basically like if they added a Qinling panda. It's an obscure, smaller, brown, rare version of something we already have for free.