What is a black spider with a white horizontal stripe and a red but?

The description you provided matches the appearance of certain spider species, such as the black widow spider (Latrodectus species). These Spiders are typically characterized by their shiny black bodies and distinctive red hourglass marking on the underside of their abdomens. However, the specific combination of a white horizontal stripe is not commonly associated with black widows, so you may be referring to another spider species or a variation. It's always best to exercise caution and avoid handling unknown spiders, especially those that are brightly colored or have distinctive markings, as they may be venomous.