Biolizard

The Biolizard (also known as The Prototype of Ultimate Life) is a creature which appeared in Sonic Adventure 2 as the final boss. It appears as a giant lizard with a life support system on its back. According to Sonic X, the Biolizard is 85 meters long, 30 meters wide, 19.5 meters tall, and weighs 87.5 tons.

Sonic Adventure 2
The Biolizard was an early version of "Project Shadow", a government project run by Professor Gerald Robotnik that was intended to create an "Ultimate Lifeform" as a first step towards humanity discovering the secret of immortality. Due to the experiments, it later needed a life support system. While the Biolizard was successful in the way that it was could regenerate and restore itself, and could self-reproduce, it was also unruly and dangerous. It was closed down when Gerald, with the aid of the alien Black Doom, produced a more advanced being: Shadow the Hedgehog.

When GUN came to Space Colony ARK in order to put a stop to Gerald Robotniks plans, they found the Biolizard and shut it down. It didn't go without a fight, however, and it destroyed almost the entire group of GUN soldiers.

The Biolizard remained in suspended animation aboard the ARK until Sonic the Hedgehog and his friends came there. After Doctor Eggman successfully achieved the seventh Chaos Emerald and put it into the Eclipse Cannon, a video of Gerald Robotnik was shown on all televisions and an audio version on the radio warning people of the Biolizard and how when the seven Chaos Emeralds came to the ARK, the ARK would plunge into the Earth's atmosphere. As the seven Chaos Emeralds were in place the ARK began to fall. Sonic and Eggman joined forces and tried to pull the ARK into orbit again by using Knuckles' Master Emerald to deactivate the Chaos Emeralds.

However, when Sonic and Knuckles entered the cannon's core, the Biolizard was awoken, determined to keep the ARK on its path to Earth. Later, Amy Rose convinced Shadow to help save the Earth, (Shadow had a flashback about his best friend, Maria Robotnik) and Shadow fought and won against the giant reptile. The Biolizard then used Chaos Control to teleport outside and fuse with the space station itself, becoming the Finalhazard, the final boss of the game. It was destroyed by the combined power of Sonic and Shadow in super form, who then used Chaos Control to place the ARK back into orbit.

The Biolizard fights by spitting dark energy balls, biting, or tailwhipping Shadow. After chasing the hedgehog for some time, the lizard stops and seems to hyperventilate, which is Shadow's chance to grind up a green tube that connects the Biolizard's head to its life support system and attack it. After damaging the system a few times, the Biolizard creates dozens of pink egg-like orbs in the air, and fires them at Shadow. Shadow must then use homing attack on the orbs in order to get into the air and attack the life support system again. Finally, the Biolizard will somehow lift Shadow himself up into the air along with the orbs, and as such he must float towards the life support system while avoiding the orbs in order to defeat the Biolizard.

After transforming into Finalhazard, the Biolizard gains the ability to fire large pink energy beams from its broken life support system, and create similar orbs to the one before, except the new ones are red and the Finalhazard creates hundreds of them to cover itself as a shield. It also fires them at Super Sonic and Super Shadow one at a time. As his life support system has been demolished, the Finalhazard has grown large pink boils all over its body, and Super Sonic and Super Shadow must charge into these boils in order to harm it.

Shadow the Hedgehog
The Biolizard was not mentioned in Shadow the Hedgehog, except in one of Shadow's flashbacks, when he remembers his apparent death after defeating it. The Biolizard was likely just an example of Gerald's work before Black Doom assisted him, and thus is of little interest to the Black Arms and wasn't stable because he didn't have Black Doom's DNA. However, it is noted that after Black Doom transforms into Devil Doom, he has an identical cry to that of Biolizard. Several other black arms have similar cries. Also, the Black Annelids, Black Worms, and larval black warriors("Death Leeches"), have a skin texture very similar to the Biolizard's. This presents the possibility of the Biolizard actually being a Black Arms experiment, but it was simply unsuccessful, or not successful enough. As it turns out, Devil Doom and Biolizard's cry is just a reused version of Perfect Chaos' cry, and the Biolizard is just a lizard that was horridly mutated. The fact that it has the same skin color as the Black Arms is probably because Sega reused the skin color for the Black Arms, and therefore might be a coincidence.

Sonic X
The Biolizard's role in the anime Sonic X parallels the one it plays in Sonic Adventure 2. The difference is that the creature is not specifically named as a prototype of Project Shadow. In the original Japanese version, Knuckles identified the Biolizard, but in the English dub, it was unnamed, and Knuckles instead simply stated that he wouldn't let the monster get the Master Emerald. When Professor Gerald's diary reveals what really happened on the ARK 50 years ago, a small green lizard was shown that could have been what the Biolizard looked like before its current state.

Theme songs

 * Biolizard: "Supporting Me" by Everett Bradley.
 * Finalhazard: "Live And Learn" by Crush 40. (Shared by SA2 as the Main Theme)

Trivia

 * During Sonic Adventure 2's development, originally Knuckles was to fight the Biolizard.
 * According to Rouge the Bat's report in the official strategy guide from Sega, there is a section that deals with the Biolizard. An interesting note is that the pink-orb objects the Biolizard produces is produced through asexual reproduction, which means that the there could have been more than one Biolizard, however this is unlikely. It is often assumed that the Biolizard, is in fact, female (due to the eggs), which would make "her" almost like Shadow's sister. The likelihood of this is disputed.
 * When Biolizard summons the pink spheres and later levitates Shadow, the roar used is the same roar used for Perfect Chaos in Sonic Adventure.