Andy Warhol and Nico were cool, they were eccentric and underground and enjoyed wearing costumes on occasion, all of which made them the perfect characters to play Batman and Robin in a photo shoot for Esquire.
The shoot happened in 1966 when Batmania was in full swing thanks to the Adam West TV show, so Nico posed as Batman and Andy as Robin for photographer Frank Bez's seriously hip photo spread meant to draw in younger, hipper readers for Esquire.
Andy was a big fan of Batman, and comic books in general, so he was probably pretty jazzed about the shoot, and Warhol's 1964 film Batman Dracula is considered the first appearance of a campy version of Batman.