Hi Sue,
Great info thank you for sharing and educating. Many agencies ask for releases on dogs and horses. Getty is one in their Macro collections. We have had to comply on several occasions.
From time to time on iStock, the same issue has come up on the forums, but I can't remember what the current 'bottom line' is, since the forum search feature is grim and searching 'pet property release' didn't help. I've had some horses accepted recently without PRs. {Of course, I've altered the blaze pattern!) But if often depends which inspector you get.
I've had a certain iStocker insist, to the point of rudeness, that they can take a photo of any animal, any time they want, without a property release, as long as they are on public land when taking a photo of the animal. I think it seems like a slippery slope, personally. But as of right now, it is my understanding that a property release is not required because you can't technically "manufacture" an animal. Which I also dispute. If you are a breeder, you are "making" those animals by pairing up the parents.