CERBERUS
Typically when you think of vicious dogs guarding a property gate, they're making sure nobody gets in. Cerberus on the other hand—or head—is in the business of making sure nobody gets out.
The famed hell hound of Hades has the tireless job of making sure the dead stay where they're put. So tireless, in fact, he's got three heads to keep a look out. (Although some depictions of him feature 50-100 heads, and varying amount of protruding snakes.) However, descriptions most consistently show the dog with multiple heads and a serpent tail. All in all, a canine creature you most certainly went to let lie. Except for one guy named Heracles.
You might be more familiar with his romanised name "Hercules," but however you know him, he's most commonly associated with his Twelve Labors. Because Heracles was the son of a mortal woman and Zeus, Zeus's Goddess wife, Hera, was understandably bent out of shape. Unfortunately, she directed much of this aggression to her philandering husband's son as opposed to the Lightning God himself. Try as she might, Heracles always managed to survive her torments. So, eventually she gave up and was like, "I don't know, go and like, bring me mythical plants and creatures and shit." In reply to Heracles's reasonable question of quantity, Hera presumably ambivalently replied, "Twelve." It's important not to dwell on this part of the story specifically, but here we are, already three sentences in! Oh, one more thing: Heracles had to do all this for King Eurystheus who was acting as Hera's representative. She couldn't even be bothered to follow through on her own punishment! Okay, we can move on.
Heracles had to go all over the fucking place to find magical plants and animals. Each one sent him on an adventure, none more notable than his journey to the underworld. Keep in mind, this was the twelfth and final labor. The dude had already slain a lion, stolen a lady's belt, cleaned a bunch of stables, captured a bull and some horses, among other things that really seem like Eurystheus was running out of ideas fast. So, Heracles is geared up to grab some more fruit or whatever, and then he's told he's got to bring back a three-headed dog from literal hell. Cool.
For this one, Heracles had to pull out all the stops and go through an Avengers-style "assembling the team montage" getting together the likes of Athena and Hermes, among others depending on who you're asking. Heracles somehow managed to capture the ferocious dog and present him to the king, who after all that, basically said, "Whoa, crazy, I totally thought that was impossible! You can take him back now." Thus climactically concluding the Twelve Labors of Heracles.
Although Cerberus is a terrifying creature, he's also often thought of as a protector of the Underworld and is loyal to Hades. And, since he's been tamed by Heracles with brute strength, by Orpheus with music and by Psyche using honey cakes, it seems like deep down, like any myth, there's more than meets the six eyes.
See you out there.