What do we know?

Medrash tanchuma parshas teruma, siman 6 brings rebbe yehudah saying that it was a one-horned, large animal with 6 colors (the baal haturim writes that tachash is gematria “שש גונא”, which means 6 colors), according to rebbe nechemia it existed bederech neis for one time just to make the yerios (curtains) of the mishkan.

In shabboa 28a-b there’s a discussion regarding if the tachash was kosher, and if it was a chaya or behema; some hold that it’s a unique creature which is neither chaya nor behema.

Rav hirsch writes that the shoresh of the word tachash is חש, that it was able to run very fast

Reb Eliezer
If I’m not mistaken, it wasn’t technically a unicorn, as unicorn is one-horned horse and Adam sacrificed a one-horned bull.

I did some research on this. The Yerushalmi Shabbos (2,3) brings an argument on this. Rav Hoshiah there quotes the pasuk in Tehilim (69,32) which is also quoted in our gemora in A’Z (8,1) indicating that the same bull with one horn is the tachash which Adam Harishon sacrificed being a unicorn and maybe it was not taken into the teiva by the mabul and was recreated for the mishkan and then hidden afterwards.