There's genetic evidence that the Omicron variant evolved in mice:

> Scientists at the Chinese Academy of Sciences now report that they have found a pattern of mutations in Omicron that is typical of a virus that has infected mice.

> They believe SARS-CoV-2 jumped from a human to a mouse in mid-2020, then back into a human in late 2021.

> They say it is unlikely that the intermediate host was a laboratory mouse.

[0] https://www.medicalnewstoday.com/articles/covid-19-did-omicr...