Anvil Crawlers are a type of lightning that can appear to crawl out of a cloud during a severe thunderstorm. In the development of a thunderstorm, an anvil cloud or towering cumulonimbus cloud will form as a result of convective processes in the atmosphere. 

During the dissipation of a severe storm, multiple discharges of lightning that never appear to make it to the ground will appear to cling to the base of a cloud. Looking up at the cloud base, an anvil crawler will look like snakes of lightning swirling underneath the clouds. (Source)