ALL snakes have scaly skin; that is a characteristic of reptiles. There are NO venomous snakes in the Dominican Republic, however. The thin black snake you saw is most likely a species of Racer, related to the ones we have in the US. Many uneducated people believe every snake they see is "poisonous"(actually there's no such thing as a "poisonous" snake, anyway), when that simply is not true, but they will always tell you to stay away from such-and-such snake because they believe it to be dangerous.
