Just chiming in here, but it doesn't confirm an Umbral Rhino. All it confirms is what was said in the Vitruvian by Ballas, that warriors chosen by the Orokin and then infected with a strain of infestation to create powerful bio-golem warriors had a tendency to go rogue. I mean, if you stretch the idea a bit, Titania should be considered and Umbral frame as her lore in the Silver Grove quest ALSO states that she was to a point autonomous. The fact that this "proto-rhino" was affected by a Zariman kid on ice simply plays on the same concept that somehow Operator "shh, It's ok..." is magical in calming down raging bio-augmented WMD's.
Personally, I hold that Umbral Excal is an oddity. He's Umbra because that might just be the name or title of the Dax that was turned into it. I also believe that only the very first frames were made using live human subjects and then promptly put to death because they were uncontrolable. And the later frames and the ones we use now are built on engineered "meat puppets". I base this on "warframe cryopods" not containing warframes, but some otherwise never seen lotus agent like character that I believe to be a sort of flesh doll designed and grown for the exact purpose of being turned into a frame. This also plays into Ordis's statement that warframes don't have memories but Umbra does, it's because Umbra was based on a person while as our frames are based on a pile of meat.