According to Roman and Greek mythology Cerberus was one of the guardians of the Underworld.
Most times described as a massive three headed dog, over time its myth took many, more extravagant shapes.

Cerberus was a vicious beast that guarded the entrance to Hades and kept the living from entering the world of the dead. According to Apollodorus, Cerberus was a strange mixture of creatures: he had three heads of wild dogs, a dragon or serpent for a tail, and heads of snakes all over his back. Hesiod, though, says that Cerberus had fifty heads and devoured raw flesh.
(Source)

As it often happens when I draw stuff for fun at home, I hadn’t exactly planned ahead.
I started with a muddy, greenish background, vaguely reminiscent of an Underworld environment, and put a big shape in it. Then I thought “hey, it could be a three headed dog guarding something”, and went down that road.

I always thought Cerberus was one of the most under-appreciated monsters of the entire Greek mythology.
He had a pretty important duty: keeping souls from escaping the hell they were confined into. I’m sure all living beings should be grateful for his efforts, lest having to share their vital space with rotting, slightly upset undeads.
Ok, he would also enthusiastically maul anyone trying to enter Hades, but I’m sure it was more likely the other way round.
I really can’t sympathise with all the heroes who kept bothering him just to prove a point.

Anyway, after a few minutes of research I decided I couldn’t be bothered to include the entire bunch of Apollodorus’ zoo into its body, but I quite liked the many legged Cerberus in front of the Royal Institute Technology of Stockholm, so I took that as a reference.
The human character came almost as an afterthought, just to set the scale, and I didn’t know what it would have been until the very end.

For some reason I eventually went for a vaguely Norse-hero-look. The two myths don’t have anything to do with each other, but anything even vaguely Scandinavian looks automatically heroic, so in a sense it fits.
Who else other than a mindless hero would challenge anything that big?
Anyway, animated gif of the process to follow.