Chameleos

Chameleos, also known as Oona, is a large chameleon-like Elder Dragon that has the ability to be invisible almost all the time, steal your items, and poison you. When it is angry, it produces a green smoke from its mouth that allows hunters to know where it is. Oonazuchi can spray a greenish-yellow liquid that drains all the stamina bars from a hunter that gets hit by it. It can also shoot a ball of liquid that can lower a hunter's defense by half. Using bombs or dragon element weapons on its head can 'Blow' off its horn and weaken it's ability to camoflague. Similarly cutting off its tail will do the same, the combination of both of these will render the Elder Dragon perminantly visible until it is slain. You can also carve off its tail.

To temporarily confuse and prevent Chameleos from becoming invisible again for a while you can sonic bomb it during an attack (you usually have some sonic bombs in supply items). After 25 minutes have passed the Chameleos can flee, therefore completing the quest without having slain it. If it has not sustained enough damage during this 25 minutes it will flee when it has. Especially with your first Chameleos it will not die first time (unless you have a really good weapon). But any damage done to it will carry over to the next time you attempt the quest, whether you are fighting it in the Swamp (MH2) or the Forest and Hills.