Are you sure that the lice on your pony are the same species as on the goats?
If your lice treatment is also licensed for sheep then you can use it at the same dosage rate for goats. Otherwise I would be careful, especially re dose. A phone call to your vet should help.
I use Cooper Spot-on, but it is quite expensive if you only have a small number of animals to treat. It does last for ages though... can be used on cattle, sheep and goats.