You have to get a reed pipe from the big music hall. The statue with the reed pipe will tell you the tune then give you it on a piece of paper. Go through the boat ride and play the tune to the beast.
the minotaur is a beast.
In the underwater cave.
at hades
you have to climb the ropes
The Hydra is a six headed beast on Poptropica.
The question is not specific enough. (see the related questions for a possible answer)
Go into Poseidon's maze, swim to the bottom left and you should find the cave.
Get under a snowball and your force field will deflect it on to him.
you make it go to sleep and shoot it in his mouth 4 times
The water fall is at the top of the beast. You have to go to the aqueducts and change the course of the water flowing down.
Use the pomegranates (from the garden of the Sphinx) to get to the Underworld. Cross the River Styx and reach the beast (Cerberus). Play the reed pipe notes from Euterpe : blue, green, yellow, blue, red, green.
To get the crown you must find the pomegranate and put it on his shrine. Then you must go through the river if styx and fight the beast and then you can get his crown