A Reo (spelled Rio in earlier media) is a relatively powerful enemy found mainly in Brinstar. They appear in Metroid, its remake, as well as Super Metroid, and in Metroid Fusion as X-hosts or mimics in Sector 2 (nicknamed Reo-X).
Reos in earlier appearances stay immobile until they see a threat, at which point they execute rapid, continuous swooping dive attacks with their talons. In later games, starting with Fusion, they fly back and forth until their target is in sight, to which they will dive towards.
Reo-X have a more gruesome physical appearance than their natural counterparts, due to the X Parasites tampering with their victim's DNA; along with the color change, they are larger in size and have far larger claws (appearing to be almost armored).
Reos can take much more damage than most other early-game enemies in the first Metroid. In Metroid: Other M, they have a new coloring and a melee attack that involves latching onto Samus and can be found in the Biosphere of the BOTTLE SHIP, often emerging from Fly Pods that resemble a giant brown compound. However, these Reos are the weakest variant ever encountered in any Metroid game, as they can killed with a single shot from the Power Beam. 
The Gerutas of Norfair act exactly the same as the Reo. The War Wasps of Metroid Prime are designed similarly to them, especially the Other M variants. Reo were displayed with the capability to wield small firearms in Deceít Du Jour.
"These are also pretty strong. They zoom down to attack Samus from the upper walls. The red ones are stronger than the yellow ones."
"Flies from the ceiling toward Samus. The red ones have strong attack power and the yellow ones are weak. Several shots with the normal beam or one Missile will defeat them."
Official Nintendo Player's GuideEdit
"He attacks Samus from the ceiling. His defensive power is high, and he is hard to kill with a normal beam."
Super Metroid manualEdit
"These creatures have sharp claws and can withstand several hits."
"They fly in diagonal patterns and look like giant moths."
"These clawed, fly-like enemies from Metroid show up in Smash Run, too. They float in midair, then fly in a U shape when you get close. They're predictable, which makes them easy to outsmart, but stay on your toes anyway—their attacks can send you flying. One way to beat them is to grab and throw them when they buzz near you. "
The PAL version of the trophy changes the spelling of "midair" to "mid-air".
"These enemies swoop down from above to tackle fighters. However, they can't actually hit you if you're directly below them. Give it a try!"
- At the beginning of Metroid: Other M, Samus will encounter a Reo while running down a hall in the Main Sector. It will latch onto her and knock her down to the floor. This encounter is unavoidable.
- Dragotix and Grippers are also sometimes seen feeding on Reos. This is similar to Gerutas in Zero Mission when ensnared by Tangle Vines.
- Artwork in the Japanese strategy guide for Metroid: Other M implies that Mellas and Himellas are subspecies of Reo, as they have arrows pointing to one another.
- This may also mean that Mellows are a subspecies as well, although they are not featured in the game.
- Metroid Fusion: The Official Nintendo Player's Guide refers to Reos as a "buglike beast".
|Food, Nourishment and Sustenance|
|Plants||Blueroot Tree • Fruit • Red Starburst • Sap Sac|
|Species||Geruta • Embers • Kiru Giru • Pirate Militia • Reo • Ripper • Sciser • Scritter • Skree • Space Pirate • Tallon Crab • Torizo • X Parasite • Zoomer|
|Specific Humans||Samus Aran • Virginia Aran|
|Carrion||"Deceased Soldier" • Amok Mommy|
|Liquid||Lava • Phazon|
|Artificial/Misc.||Elite Nutrient Mix • Energy • Nutrient-grade biomatter • Synthetic Metroid rations • Wrecked Ship|