In Quake, a Scrag is a kind of floating creature resembling a white Cobra snake combined with a human torso that spits small yellowish green globs of poison spray at their opponents. These globs can last up to ten seconds if they do not collide with any entity or surface. In some places, the Scrag is known as a Wizard (for example, the Scrag model is stored in a file "wizard.mdl" inside PAK0.PAK). This is perhaps due to being influenced on the HP Lovecraft creature called the Wizard. The Scrag floats by means of a gas bladder. If the Scrag is killed, so is the bladder, causing it to fall to the ground.
Alone and with other Scrags, a Scrag is easy to kill. Two Super Shotgun blasts or one rocket can kill it. To avoid its projectiles, just strafe around it. If there's a group of them, take out your Nailgun and wipe them out.
With other Enemies, Scrags are a major pain. They usually pop up in front of you at inappropriate times, such as when you're about to fire a rocket at another enemy. In larger rooms, they tend to stay away from a fight and take potshots at you. The best way to minimize them from interfering is to shoot them when the enemy you're fighting is stunned. When the enemy is stunned, move back and take out the Scrags with one of your Nailguns. Or, you can try and get the Scrag caught in the splash damage of a Grenade or Rocket, but this can be risky, especially if you're in a tight room.
As the Scrag continuously spits poison blobs, it can easily provoke monster infighting. However, its flight capabilities often means that melee-only monsters may not be able to retaliate against the Scrag, nonetheless the Scrag is still useful for distracting enemies and getting them off your back.
The Scrag is the only flying monster in vanilla Quake and it has an axis of movement similar to a helicopter such as being able to hover. The Scrag isn't fast but it can get to where it needs to in order to attack, including getting in and out of water.
Also going very close to the Scrag will make it unable to move and unable to attack the player for a few seconds, The Axe can be a good weapon in these situations but it can be risky if you are low on health.