Set in ancient Rome during the reign of the Roman Empire, the story is told in episodic arcs depicting death matches in the Colosseum, and centers on the life of the last living wyvern, Durandal, and his relationships with various arena fighters over the years.

The story skips to fifteen years later, where Fin is shown to be a strong and undefeatable gladiator, ensuring his survival.

The cruel emperor, Domitianus, arranges a death match between Durandal and Fin, promising freedom to the winner.

Masasumi Kakizaki stated that he always liked Rome from the Roman Empire era and wanted to make a work based on it.

[13] Writing for Le Figaro, Valentin Paquot described Bestiarius as "clever mix between Gladiator and The Lord of the Rings", adding that Kakizaki graphically reinvents mythological creatures, giving them a dark and terrifying appearance of realism.