Sounds VERY much like lice.
The pony I just got is infested with these critters, I used a pour-on treatment which is a biological control, it works more slowly than an insecticide, but will be effective for 3 months without the need to re-apply.
Not sure if it is available where you are.
The active ingredient is Triflumuron which interferes with the formation of chitin in the lices shell, therefore killing it when it moults. (They usually moult every 10 days, until maturity at 30 days or s, and once in the egg before they hatch)