The horse's feet still show the foliage and I'd like him a little darker, but still showing the detail.

[Copy Merged] and [Paste] into your image.
[Image] [Adjust] [Hue & Saturation] Set Saturation to -100 to get a black and white image.
[Image] [Adjust] [Brightness & Contrast] Adjust to get the horse the right darkness but with enough contrast to see his muscles and contours.

[Ctrl and click] on previous cut horse layer to get a mask. [Select] [Inverse] and then on this layer, [Cut] to leave just your altered horse. The previously green foliage is now white/grey and blends more with the water splashes.