'Fraid not, but I have promised my boy a cake for his 18th! So if you find a good recipe please let me know .
When I was younger, I used to make him Christmas dinner which basically consisted of sliced carrots and apples mixed in with a couple of Weetabix and hot water, plus one or two sugar cubes on top. (The most un-Christmassy meal ever, I know, but it was all we really had in the house that he could eat!). He seemed to like it but hasn't had it for donkeys' years so I may do that again for him this Christmas.
He's a big fan of ginger biscuits, so I might just top them with some sliced carrots/apples etc. and tell him they're cupcakes!
We made one for one of the horses on the yard who was turning 20. Just took his normal feed, soaked it, added some molasses and stuck it in a cake tin with a removable bottom. Then really compressed it and left it 'set' for about half and hour, took the bottom out with cake attached and ta-da! (can decorate with Polos, carrots, apples etc)
A good place to find proper recipes are the american forums like 'horse grooming forum'
I'd love to find a good recipe! My horse gets birthday cards from the family, presents, a large xmas stocking filled with pressies and an advent calender. I unwrap her pressies with her at xmas too..