Daisy

Daisy was 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. The preceding text mentions that "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, this would appear to be an example of retconning.

Other appearances

 * Daisy makes an appearance in Action Doom in the final level, which takes place at Deimos due to unforeseen events that took place at the "Union Aerospace Laboratory". She is animated, and runs around Doomguy's base in space; the player cannot harm Daisy in any way.
 * She also appears as a cheat skin of past versions of Skulltag.