I Am Legend's original screenplay featured a different fate for Neville's beloved dog Samantha. I Am Legend was the third adaptation of Richard Matheson's classic 1954 vampire novel, following The Last Man On Earth with Vincent Price and Charlton Heston's The Omega Man. 2007's I Am Legend followed Will Smith's Robert Neville, who is seemingly the last man alive following a virus outbreak that wiped out most of the planet and turned the rest in mutants called Darkseekers. While most of the movie is a one-man show for Smith, that's forgetting the all-important contributions from the character's dog Samantha, played by a German Shepherd named Abbey.
Samantha is by Neville's side for the first two acts of I Am Legend, serving as his only real friend in post-apocalyptic New York. She is also involved in the movie's most heartbreaking moment, where she's bitten by infected Darkseeker dogs to save Neville, and despite his best efforts, he has to euthanize her when she turns. Sam is based on a passage in the original I Am Legend book when Neville discovers a stray dog is wandering around his neighborhood, but despite his attempts to save the dog it later dies.
Giving Neville a dog to act as a companion came from screenwriter Mark Protosevich in his original draft. Protosevich's 1995 I Am Legend screenplay was once considered one of the greatest sci-fi movies never made and it bears some big differences from the final movie. It took place in Los Angeles, had much larger setpieces and a completely different finale. The movie ultimately slipped into development hell, and while the final product features many elements from Protosevich's draft - such as the Darkseekers lying a trap for Neville - it has notable differences too. This includes the fate of Neville's dog - who is male and unnamed in the script - where instead of euthanizing the main character lets him go. The dog later returns in the final scene where Neville and other human survivors are escaping to a pier, and Neville's dog now leads a pack of mutant canines.
Of course, this is a moment of tension as it appears Neville and the others are going to be ripped apart after just surviving a showdown with the script's main villain. However, this I Am Legend ending offers a more hopeful resolution, where the dog recognizes Neville and forces the other infected animals to leave. He then lets Neville escape and "mournfully" howls before running back to his pack. Protovesich claimed in a Money Into Light interview that people often complain to him about Samantha's death, but as his original screenplay revealed, this wasn't his concept.
Akiva Goldsman later re-wrote the I Am Legend script and added Sam's death, while Protosevich had a different fate in mind. Both Neville and his dog survived, but the latter had evolved into a new form of life. The fact the dog spared Neville was meant to be a sign there was hope that what was left of mankind could co-exist with the hemocytes - the name the Darkseekers were given in this '95 script - but the released movie ultimately went a different route.