He is an alien that grows when people lie. He tempted Junior to lie, simply by saying "A little fib couldn't hurt anybody." Fib continued to grow as Junior lied until he was big enough to rampage through the city. After which he turned on Junior and tried to eat him and LarryBoy.
LarryBoy did not defeat the Fib, he was squeezed until his plunger popped out and almost eaten alive. Eventually, he did help Junior down from the water tower with his remaining plunger ear.
In the end, he was defeated by Junior telling the truth before he could eat Junior and LarryBoy.
After Junior told a few lies, Fib became gigantic and decides to destroy Bumblyburg, and holding Junior in his clutches.
Suddenly, Junior discovered that Fib had only become Junior's enemy.
Once Junior realized that he's the only one to stop it, he told everyone the truth, which caused Fib to shrink and disappear into non-existence.