Kamen Rider Dragon (Tentative)
The belt hatches the eggs.
If the monsters doesn't absorb human energy they will revert form. Grown up beast -> Young beast -> Egg.
The protagonist uses pet monsters to transform into a grown up beast. They have a master-servant relationship.
The monsters are similar to Ryuki's, they don't possess the user, they give a part of their power. This is what affects the form and fighting style.
By combining the eggs, they can be changed into a bike or a giant monster.
Because of a curse from the Mazoku, legends all over the world were turned into monsters.
Note, "monster" was always written as モンスター(monsutaa) in the original post. It's not referring to Mazoku. Mazoku and monster are 2 different things.
This was written from Yobuko as well.
http://netarider.blogspot.com/2011/03/kamen-rider-2011-2012-kamen-rider.html