Prerelease and unused content in Animal Crossing
There are several beta items and "glitch" items in Animal Crossing. These are items left in by the game's developers, intentionally or not. They were never intended to be seen by players, but through hacking or by chance, they can be acquired.
As these items were not meant to be a part of the actual gameplay, they were not added in Animal Crossing: Wild World or Animal Crossing: City Folk.
DUMMY
The DUMMY is a beta test item that looks like a floating white triangle with red Japanese letters spelling "ダミー", or "dummy", on it. Since it is a beta item, it can not be sold nor does it appear on the catalog.
It can be gained by talking to villagers inside igloos during the winter. The animal might want to play a game where he or she will ask you to pick two cards. One card you have to buy something and one card you will get a free prize. Sometimes, the prize card will be a DUMMY. If it is, hopefully you will pick the prize card and get the DUMMY.
There are several other beta items in Animal Crossing with the exact same appearance as the DUMMY, including the "unused monkey" and the "unused dresser". The index number for the DUMMY is 33C4.
Bluefish/deadfish
The bluefish is an item that can sometimes, but very rarely, be given by Gulliver after he washes up on the beach, or found buried by a villager. If a villager buried it, they will post a message on the bulletin board, showing glitched text where the item's name would normally be. It looks like a blue fish with a hook coming out of its mouth. Using hacking devices such as the Action Replay appears to cause this; it is unknown whether it would happen without the use of the devices themselves.
If found buried, the player can dig it up, and they will receive the item, along with the usual message stating that they dug it up, but with the same glitched item name. If the player drops it back on the ground, they cannot pick it back up, and the acre will likely freeze or reset the game whenever they try to enter it again. It can only be safely disposed of by giving the item to Tom Nook; he will not pay anything for it, however.
The sprite for the bluefish is in fact the one used if a fish is placed on the ground through hacking. It's likely that the developers originally planned to allow players to drop fishes on the ground, and the sprite was made so the players could see the fish on the ground, but the decision was later reversed. Fishes left on the ground are not the bluefish, and are therefore harmless. In addition, they can be picked up.
Glowing yellow block
The glowing yellow box appears as a glitchy square with no name in your inventory, but when dropped and when the player walks into another acre and back, there will be a strange, floating, and yellow box. It oscillates from left to right while rotating counterclockwise slowly. After a bit, it glows a very bright yellow, then dims back down. It pushes the player away if they stand in its way, and if they use the jump code to jump and land on it, they will stay on the block while it moves back and forth.
It is unknown what the yellow block was used for. Some have guessed it has to do with the lighthouse, since it flashes a bright yellow color and dims out. Alternatively, it may have been used by the developers to test in-game physics, as it pushes the player character away, and it would have to be explicitly programmed for the player to stay on the block without falling off.
