Doom Eternal XP (opens in new tab) is a mod that, among other things, is transforming Doom 2 into its latest successor. It's a mod collection that initially started out as a XP mod before its creator Officer_D started collecting and tweaking Eternal mods and putting them all in one place. It now has a dog.
The dog, who you'll have to name yourself as it doesn't seem to have one, can be summoned from a sphere, like a Pokémon, and will follow you around levels. Watch out, however, as enemies and stray bullets can prove fatal. It's a handy hound, however, as it will leap into battle and snap at demons. What a brave boy!
Despite Doomguy's tough exterior, he does have time to pet his little killer. And between killing demons, he's also taught the little guy to sit and heel. It's a little island of pleasantness amid all the gore and carnage and flamethrowers (opens in new tab).
Dogs first appeared in Doom by way of Wolfenstein 3D. Sprites and sounds from the original hounds were used to create pooch pals for the Marine's Best Friend mod. Speaking of Wolfenstein's guard dogs, they've been given a friendly overhaul thanks to the Woofenstein mod. It removes all the Nazis and lets you pat the dogs until they fall asleep.
You can pet the dog in Doom Eternal XP now.