I had heard of the tsopo before and determined to look further into the matter. The animal goes unmentioned by Eberhart and Newton, but was and possibly still is part of Tibetan belief. There are two other Tibetan words for a unicorn, serou and kere, which probably refer to the same beast.
Naturalists to date have failed to track down any genuine unicorns in Tibet, but there is a kind of antelope called the chiru (Panthalops hodgsinii) and it is thought that this may lie behind the legend, especially as it was not unknown for Tibetans to cut off one of the horns from this beast. They would then sell it.
There may also be some confusion due to naming with hybrids between cattle and yaks. The male of these is called dzo, also spelled dzho, zho and zo. The last spelling is a boon to scrabble players. The female is called the dzomo or zhom. These names may have caused some confusion in passing on information to 19th Century travelers.
Incidentally, in English a yak-cattle hybrid can be called a yakow,
while, if you cross your yak with a bison, the offspring is a yakalo.
On the whole, it seems likely that in the chiru we have the origin of the tsopo, but we cannot say so with absolute certainty.