If the horse is on a separate layer than the sky, highlight the horse layer.
Use the elliptical selection tool at a fixed size or a fixed ratio (1:1) to make a round selection.
Select > Transform to adjust the selection's size and move the selection to the desired location and press Enter.
Click Delete and the sky will show through the circle.
If the horse is not on a separate layer, make a round selection (as my previous post) of the sky.
Right click on the selection and click Layer Via Copy.
Select the new layer and move the selection to the desired location on the horse.
Oh my goodness!! You, Peru Bob, are a wonder! Thank you so very much. I can now finally go to sleep.