The Druid can use their Wild Shape Feature to change into Monstrosities in addition to the Beast.  There are limitations however.  The Monstrosity cannot:

• Have an Intelligence Score Higher than 6.
• Have Legendary Resistances or Actions.
• Have Energy-based Attacks.

Just as with a Beast, the Druid must have personally witnessed the Monstrosity. Additionally, using the Wild Shape Feature in this Way cost the druid two wild shape uses instead of one.  The Monstrous Form last half as long as a normal form would and all the normal stipulations for that druid apply.  (ie. CR Limitations and if they cannot Wild Shape into a Creature with a Swimming of Flying Speed this still applies.).