The ancient Egyptians used lions in their art. So did the Mesopotamians and the Greeks. Islamic calligraphy often includes lions, as does the art of Southeast Asia — though lions were never even native to the region. Sometimes lions have stood for courage and strength; sometimes they have added a sense of majesty to buildings. Here in the United States, the National Zoo has two notable 5,000-pound brass lions flanking an entrance, and the Botanic Garden in Washington, D.C., features four bronze lions crouching on the American flag. Turns out, Boston is crawling with lions, too. In the North End, at the public library, in Chinatown, in Dorchester, at the MFA. Once you start looking for them, they seem to be everywhere.