Marilyn Monroe in 1948 playing with her chihuahua Choochoo, who takes a mighty leap upward for a treat.
Marilyn never bothered to housebreak Choochoo so told everyone the dog was incontinent rather than confess she never trained her properly. Many chi owners have this problem. Chihuahuas easily learn to ask to go outside to pee, or will pee on newspaper, but they must be trained to do it as they're also smart enough to know if you don't make 'em do it a specific way, then they can pee wherever they like.
An episode of the cartoon series Top Cat was "Choo Choo Goes Ga-Ga" (1962), which parodied Marilyn Monroe as Marilyn Maroon, upon whom Top Cat's right-hand-cat Choo Choo had a crush. Whether the series character Choo Choo was named for Marilyn's chihuahua is not likely but not impossible either.
A lifelong animal lover, other of Marilyn's pets included her childhood dogs Jippy and Tippy, a Persian cat named Mitsou which she had in the 1950s, a parakeet named Butch, a setter named Ruffles (circa 1947), and a bassett hound named Hugo (circa 1956).
Arthur Miller kept Hugo after the divorce, though reportedly Hugo liked Marilyn best, and she missed Hugo more than she missed Arthur. Frank Sinatra seeing her grieving for her separation from Hugo gave her a little white poodle, which he obtained as a pup from Natalie Wood's mom, and which Marilyn named Maf.
Andrew O'Hagan's The Life and Opinions of Maf the Dog and His Friend Marilyn Monroe (2010) is a delightful fantasy-take on Marilyn's life from the p.o.v. of a dog. The cartoon embedded below is a trailer produced to advertise the novel.