It's called a roll cloud -- low, horizontal, tube shaped, and completely detached from the cloud base near it. They are rare to see but when present they are located on the leading edge of a line of thunderstorms, cold fronts or squall lines.
Roll clouds form when cool air sinking from a storm cloud's downdraft spreads out. This is called a gust front. This outflow undercuts warm air being drawn into the storm's updraft. As the cool air lifts the warm moist air water condenses creating this kind of cloud, which rolls with the different winds above and below.
While they look like tornadoes turned sideways, they are not and do not produce tornadoes.