The Mountain Monsters are monsters created by Hex. They are very tall, like Fourarms, (about 10 Feet Tall) There weaknes are sonic waves, like used by Benwolf, Echo Echo, or Mountain Hands. They were only seen once throughout the series, due to been crushed to bits by Ben.
They were relapsed back to the rock creatures.
They look similar to the rock creatures, Only because they are both made by Hex and Charmcaster.