Beta items in Animal Crossing (GCN)
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.
The bluefish, also often called a "deadfish", 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 fish on the ground, and the sprite was made so the players could see the fish on the ground, but the decision was later reversed. Fish left on the ground are not the bluefish, and are therefore harmless. In addition, they can be picked up.
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 only in the US version of Animal Crossing by talking to villagers inside igloos during the winter. The animal might want to play a game where he or she will ask the player to pick two cards. One card the player has to buy something and the other card the player will get a free prize. Sometimes, the prize card will be a DUMMY. If it is, hopefully the player 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.
Glowing yellow block
The glowing yellow box appears as a glitchy square with no name in your inventory, but when dropped in a location and when the player exits the acre and returns, there will be a strange, floating, and yellow box in its place. It oscillates from left to right while rotating slowly, counterclockwise. 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.
The index number for the glowing yellow block is 8003.
The paper airplane is an item that duplicates when placed outside. In the player's pockets, it appears as a black box, but once dropped, if the player leaves the acre and returns to the spot, it resembles a paper airplane.
When the player enters and exits a building, it will double, causing the number of paper airplanes to increase exponentially; there will be around 32 paper airplanes after the player has entered a building five times. Upon pressing "A" while standing over the object, it will visually disappear, but it still exists in the system's memory.
It has some beta functionality left in the game. After the player presses "A" while standing on the paper airplane they can press and hold "Z", then flick the control stick up or down to throw it. Once a building is entered and exited, the missing paper airplane will reappear and double. If too many of the item are in one acre, the acre itself will crash and be rendered useless. In extreme cases, the paper airplanes can spread to every acre in the town and render the town unplayable.
The glowing yellow block pushes the paper airplanes away, but it only affects them visually and does not affect their position in the game's memory. The paper airplanes can also duplicate the yellow block if placed close enough.
The index number for the paper airplane is 8000.
The sickle is a beta item left in the game's programming and in the Animal Crossing guide. When obtained through hacking, it can be found in the player's pocket and appears as a bug net. When equipped, it looks as if the player is holding nothing as well as acts like they are holding nothing. However, on the pocket menu when the cursor is over the player, the game shows that they are holding it.
When dropped outside, it appears as a toolbox -- the sprite used for all tools when dropped outside in the original Doubutsu no Mori. While some outlets have theorized that this is an indicator of Animal Crossing carrying over this trait in early versions of the game, it is also likely that the toolbox sprite simply appears as a placeholder, being otherwise unused in the final game. When dropped inside a house, it appears invisible due to the lack of an appropriate model, but it can still be picked up.
The index number for the sickle is 222C.