There have been past examples of animals in zoos, most recently tigers in Indonesia, also catching COVID-19 (https://abcnews.go.com/Health/wireStory/rare-sumatr

There have been past examples of animals in zoos, most recently tigers in Indonesia, also catching COVID-19 (https://abcnews.go.com/Health/wireStory/rare-sumatran-tigers-recovering-catching-covid-19-79199708 https://abcnews.go.com/Health/wireStory/rare-sumatran-tigers...). However it is scarier to see wild animals carry COVID, especially at such high rates. Have there been examples of past viruses that transmitted from humans to animals, mutated, and then spread back to humans? What I wonder is if a much larger number of hosts exchanging the virus means a higher probability of a dangerous mutation whose evolutionary fitness is adequate to compete against even the Delta (India) variant.