There are 100s of Coronavirus in animals. Only 7 so far identified in humans. The first 4 discovered in 1960s and related to the common cold. The last three? SARs, MERs, and now this one, SARs-2
It spilled over from an animal and what a doosey it is. It ain't rare.
https://www.npr.org/sections/goatsandsoda/2020/02/20/807742861/new-research-bats-harbor-hundreds-of-coronaviruses-and-spillovers-arent-rare