David's Treasure Zoo is a room randomly generated in the Dungeons of Doom. Most of the squares in the room are occupied by a sleeping random monster, and a random amount of gold. Zoos can be randomly generated on each level in the main branch, starting with level 7. Each eligible floor has a one in seven chance of containing a zoo unless the level already has a shop, a throne room, or a leprechaun hall, or is a maze or a special level. In practice, this means the player has about a 10% chance of finding one below DL 7.
The top floor of Sokoban always has a zoo, with an amulet of reflection or bag of holding as a prize in addition to its usual features; the item sits on a cursed scroll of scare monster with "Elbereth" engraved on the tile. Another guaranteed zoo occurs in the middle level of the Wizard's Tower. Fort Ludios will always have a zoo, assuming it is accessible during a given game. In addition, the Tourist quest's Thieves' Guild Hall has a zoo as well.
Often, it is safer to displace several pets into a zoo, rather than fight it out yourself and risk being surrounded by multiple attackers. Monsters will use their ranged attack only against you, and will only attack the pet when it attacks them; you may need to stay close to heal your pet if possible.
Another option is to use stealth, whether via intrinsic (e.g. playing a rogue) or extrinsic (e.g. wearing a ring of stealth, elven boots or an elven cloak). This will allow you to take on each sleeping monster one at a time, making combat much easier. Be careful not to let your pet too close if doing this, as they make wake some of the sleeping monsters. In addition, be wary of kicking objects, as well as engaging exploding enemies like yellow lights and gas spores - the sounds will inevitably wake some of the sleeping monsters, which can prove potentially disastrous in Sokoban if the prize is an amulet of reflection and one of them manages to pick it up. This will not work for the Fort Ludios zoo, as the alarm will sound upon entering the portal, instantly waking up all monsters on the level.
For combat situations where most or all of the zoo's inhabitants have been awakened, you can opt to use a ring of conflict and turn them against each other. Keep in mind that you are still an eligible target for attack, and may still find yourself on the wrong end of some projectile attacks; conflict will also endanger your pet and/or cause them to attack you as well if they are in the area.