Leche de Pantera is a popular Spanish cocktail made with condensed milk, gin and water/ice. It was first created around the 1920s.
The cocktail disappeared for decades before being revived by college students in the 1970s - regaining popularity at a bar called La Barretina. It became popular as it was cheap and could be pre-made.
A bar across the street named Tasca El Corral started making a less potent pink version called Leche de Pantera Rosa (Pink Panther's Milk).