You could perhaps get a tye dye top some flares, or a t-shirt with some funky colours on or peace signs. Then get a nice big afro style wig maybe and a leather hat and jacket to go with. ( I could be totally in the wrong decade but that's the idea I got for 70's dress was like from google.) All this could be totally ott though. Or perhaps do punk 70's 
Or you could go as like 70's hippie style, this would work well if you have long hair. Just get some flowers tied into horses main and tail, If you can somehow, get a peace sign around his neck and perhaps stick one on top of his brow band. Then all you need to do is make your hair really long and straight, get a nice flower head band, wear a tye dye t-shirt and put a peace sign necklace on, and then put some flared trousers on Pm me if you need more ideas 
Wrong decade for flowers, tye dye is ok, why not do glam rock? Lots of satin, flares, pastel colours and paint your horse! Look at old videos of Slade, the Sweet, T. rex etcPrehaps get a neon coloured blanket of some sort and sew some like hippy flowers/bright flowers onto it and put it on his back or put some flowers in his mane. And attach some to his tack. Or put some peace signs on him somewhere
