In order to transform yourself into a monster you need a Polymorph Marble.
You can drop those random from different monsters.
You can't transform yourself into a monster which has a higher lvl than you.
You can increase your polymorph level with Polymorph Books. There are 3 different types of books.
Should you have the polymorph skill on P the duration of a Polymorph Marble will be 30 minutes!