You could try Nettles .. Nettles contain Histamine + Formic Acid and they are FREE!!
Simply pick a few leaves,put in a jug add boiling water as if making tea and let it steep,when cool enough apply to your skin + to the Ponies.
If there are no nettles around you can buy nettle teabags in Holland + barrett or other health stores.
You can also freeze the leaves to use through the winter months,i do when im handling lots of Hay to feed the sheep.