A scimitar wielding monkey.
Using the full Dharok armour set on low health is an effective method of killing monkey guards due to the prayer altar nearby. Due to the altar, you can use both the protect from melee prayer (which is required) and your best offensive melee prayers without worrying about prayer potions. However, there are small poisonous spiders in the area, so taking several super antipoison potions may be somewhat helpful.
If caught as human in Ape Atoll, They will pop up and put you in jail. When this monster's health gets low it will automatically heal itself. Unlike most other monsters, it is impossible to walk straight through monkey guards, so staying close to the prayer altar is recommended so that you do not run out of prayer points.