Daisy

Daisy is the Doomguy's pet rabbit.

The rabbit's head is shown in the end picture of Episode 3: Inferno, impaled on a pike as evidence that the hellspawn have come to earth, but the rabbit is not specifically identified as the Doomguy's pet.

In the end picture of Episode 4: Thy Flesh Consumed the rabbit's head reappears in the Doomguy's hand. It is here that the game mentions "someone was gonna pay for what happened to Daisy, your pet rabbit". Due to the lack of any previous mention of this in the Episode 3 text, and it being incredibly unlikely that the Doomguy would arrive back on earth in a random field to find his pet rabbit dead, this would appear to be an example of retconning.

Other appearances

 * Daisy also appears in Action Doom in the final level. She is animated, and runs around the Doomguy's house.