I had this idea I wanted to use for my dd, but she's too old now and I would love to share it in case there are any other horse lovers out there!
Imagine a white picket fence - but maybe shorten it to about 2ft high, make it as wide as the bed. Then take the (yes this will sound awful) heads off of the stick ponies that the kids play with and mount them on the wall above the fence so the horses are looking over the fence. Paint the wall behind it like a bit of a mural, with green trees or whatever, just a splotch of colour around the horse heads.
Doesn't have to be a headboard, could be just on the wall as "art."
DD thought it was an amazing idea, but she's 15 now and we never got to making it. She moved up to the attic so her walls are pitched since she's right under the roof and to have the horses hanging over her head leaning in like that would be kind of creepy. lol.