You will see them before you have unpacked your bag: small, sturdy horses grazing on the verge of the main road, drinking from puddles, occasionally trotting in single file down the centre of a residential street. They are the descendants of Spanish horses left behind centuries ago and have lived wild on the island ever since, with no natural predators and almost no fences.
Mornings around the Esperanza-to-Isabel road and evenings on the refuge tracks are the best times to see them. Do not feed them, do not try to touch them — they are not pets, they kick, and several tourists are bitten or trampled each year. They also like to stand in the middle of unlit roads at night, which is the reason every Vieques rental car has a "drive slowly" sticker on the dashboard.