Twinbellows

Twinbellows the Ferocious is a large, two-headed dog, debuting in the original Kid Icarus.

Acting as a watchdog to the Underworld Army, Twinbellows serves as the boss of Chapter 1.

Physical Appearance
In the original Kid Icarus, it appears as a large, bright red, two-headed dog with fire around its head and running along its back.

In Kid Icarus: Uprising, it appears as a two-headed dog like before, only now it is darker red with more flames engulfing its body. It has a large metal brace with a broken chain attached to its left paw, with similar-looking spiked collars around its necks. In addition, Twinbellows has a large bone structure protruding from its back, with funnel-like spouts that shoot flames running along its spine.

Kid Icarus
Twinbellows is the Gatekeeper of the Underworld Fortress, guarding one of the three Sacred Caskets. Twinbellows travels slowly, spewing fireballs at Pit; these fireballs generally pass overhead and Pit's opening to attack is when Twinbellows is gearing up for another shot.

Chapter 1: The Return of Palutena
When the Underworld Army starts attacking That First Town, Pit is sent in to aid the humans.

After fighting off Underworld troops, Pit finally arrives at a large coliseum, where he must take on the Underworld's watchdog, Twinbellows. Once defeated, Twinbellows bursts into light, and its remains are teleported away.

Chapter 9: Medusa's Final Battle
A fake version of Twinbellows appears as one of the bosses of the chapter, created by Medusa.

Battle Style
In Uprising, Twinbellows will breathe a stream of fire at Pit, which can be avoided by moving to the beast's side. Twinbellows also shoots smaller fireballs at the player, which is more difficult to avoid in this manner, so dodging is best here.

Finally, Twinbellows will lunge, and launch itself at Pit. It will strike with its claws while lunging forward, and as it reaches the wall of the Colosseum, turn around and push itself off the wall to launch itself again. It will do this for a while, but eventually tire itself out and fall over, leaving itself open to any attacks. It seems that attacking either of Twinbellows's two heads will do more damage than normal.

Twinbellows the Ferocious
''A two-headed dog (hence the name) that guards the Underworld. Medusa unleashes Twinbellows during an assault on a human city, using magic to super-size the bicephalous puppy. But, for Pit, the louder they bark, the harder they fall.''

Twinbellows the Ferocious
''Twinbellows is a flame-engulfed, demonic, doglike beast unleashed by Medusa during her attack on a human city. Its roar heralds a crimson fire that consumes everything in its path. If you hear both heads roaring, prepare yourself—this beast is definitely drawing near.''

Trivia

 * Twinbellows seems to be inspired by Orthrus, the double-headed guard dog owned by Geryon in Greek mythology.
 * However, its Japanese, German, French, and Italian names are derived from Cerberus, the three-headed guard dog of the underworld in Greek mythology.