Not sure if that person was me, but reposting a recent comment I made to that effect anyhow. The estimate and the word "epidemic" both originate from a DOJ report of all things.
I believe the DOJ report is linked somewhere in that article, if not I'll dig it up if requested.
Cops in this country kill so many dogs each year that a specialist at the Department of Justice’s (“DOJ”) community-oriented program services office says it has become an “epidemic.” The DOJ estimates that around 25 to 30 dogs are killed by cops every day, with some numbers as high as 10,000 per year. The totals could, in fact, be higher, since most police agencies do not formally track officer-involved shootings involving animals.