At a routine examination this circular mass is noted in a pony’s eye. The owner trawls the internet and is worried about it being an iris melanoma. Can you assuage her fears?
This circular lesion with a totally different appearance to the surrounding tissue, is almost certainly an iris cyst. In the horse these are generally attached to the iris while in the dog they are free floating. In each case they are not a solid mass but a fluid-filled thin-walled cyst, as shown in this ultrasound scan, taken from a different case but illustrating the same appearance.
Some would advocate lasering the cyst, but unless it is causing a problem I would leave well alone!