Cerberus

Summary
Cerberus is an animal-like monster that strongly resembles a dog from Naofumi's world. The monster is pitched black and wields three individual heads.

The Cerberus appear out of the serval rifts in the sky during the First Wave and attacked Raphtalia's Village. The Cerberus was the Wave Boss of the First Wave and was never defeated causing the Cerberus along with other monsters to completely destroy Raphtalia's village and kill most of the villagers resigning in her village.

The whereabouts of the Cerberus is unknown after the events of the First Wave and its unknown whether it is alive or deceased.

Debut
Anime: Season 1, Episode 2

Manga: Volume 1, Chapter 4

Light Novel: Volume 1, Chapter 16 (mentioned) & Volume 1, Special Extra Chapter Two (actual)

Web Novel: Chapter 19

Trivia

 * The Cerberus killed Raphtalia's parents in front of her eyes causing her to develop a trauma of blood and dog-like monsters.