A small gorilla wearing a white shirt that says "Jr." on it.
Donkey Kong Junior, in its original form, is the sequel to Nintendo's breakout hit, Donkey Kong. Developed by Nintendo and released in 1982, the arcade version of this game is notable in the fact that it uses the same cabinet as Donkey Kong, and that there exist conversion kits to convert Donkey Kong or Donkey Kong 3, and it uses the same special Nintendo monitor as the Nintendo Vs. Unisystem titles. (There is also a reasonably well-known hacked conversion kit, originally designed by Scott Brasington, to play both Donkey Kong and Donkey Kong Junior on one cabinet, known as Double Donkey Kong.) The game is also noteworthy in that it's Mario's first (but not his last) appearance as a villain, and that this game is where Mario got his name. (Before this, he was called "Jumpman" in Donkey Kong.)
The arcade board apparently isn't difficult to acquire, and both the Donkey Kong and Donkey Kong 3 boards can be modified to play Donkey Kong Jr. However, the controls and screen were special (it used the Vs. Dualsystem hardware), so they can be difficult to replace if damaged. MAME emulates this game flawlessly, and the ROM dump is easily obtained.
The premise of the game is so: Mario has captured Donkey Kong, and it's up to Donkey Kong's son, Donkey Kong Jr., to retrieve the keys to Donkey Kong's cage and rescue his father. (I do not use DK at any point to refer to Donkey Kong or Donkey Kong Jr., as DK is another gorilla entirely, and is actually Donkey Kong Jr.'s grandson. More on this below.) To do this, Donkey Kong Jr. has to climb vines and chains (he climbs up faster holding two vines, and shimmies down more quickly holding one) while evading birds and crocodiles. It's amusing, in an early-80s arcade way, but it wasn't a revolution at the time, and there are/were better games in the genre, both then and now.
The game has been ported to a remarkable number of platforms. Here's a list of all of the ports of the arcade version of Donkey Kong Jr. (not the Game and Watch version - more info on that below)
There was also a VIC-20 version completed (by Eclectic Games) but never released by Atarisoft. Info on this is scarce, but confirmation of this port's existance can be found at http://www.teamhightower.com/eclecticgames/.
The NES version (and by extension, the e-Reader and Animal Crossing version, as they are simply ports of this version) offer a "Game A" and a "Game B" at the main screen. These are simply different difficulties, as is common in ports of Nintendo arcade games to the NES. Game B was harder.
As for known bugs and design flaws, the game has only a few. (All of these are in all of the Nintendo versions save Donkey Kong Classics, and in the Atari version. I don't know if they are in the Coleco version.)
- In the second stage (and possibly the others), getting hit by one of the birds at the same time as touching the final chain will result in the game locking up, music still playing.
- The it's possible to get stuck on the little "step" where the two top platforms meet in the first stage.
- Jumping over enemies on a flat platform will net you 100 points, with the inexplicable exception of the blue alligators in the first level.
- Whether or not Donkey Kong Junior dies from falling too far is extremely inconsistant.
- In the second stage, it's possible to hop on the springboard and just barely miss the moving platform, causing you to die from the fall, whereas if you miss the platform, you won't die when you land.
(Donkey Kong Classics fixes the first two bugs, but the hit detection for the red alligators is changed to the point where they can kill you without touching you, and the red sparks in the third stage can kill you if you jump while they are above you.)
Donkey Kong Junior is also a title in Nintendo's Game and Watch series of self-contained portable LCD games. Released on October 26, 1982 in the Tabletop line (and later rereleased on October 7, 1983 in the Panorama line and on August 6, 1998 in the Mini Classics line), the game was fairly simple, but required 4 playthroughs to complete, before all 4 keys were obtained and Donkey Kong was freed. (The premise was much the same as the arcade version.) The gameplay is fairly sophisticated for a pocket LCD game, and is up to the high Game and Watch standard.
The Portable Classics version can be obtained with a little effort in searching drugstores and whatnot (as of this writing, there's one in the Walgreens that employs me), and the original Game and Watch version is available whereever collectors gather to trade such things, like eBay. (LCD games cannot generally be emulated, only recreated.) In addition, the game was released with graphical enhancements as part of Game & Watch Gallery 3 and Game & Watch Gallery 4 (released December 7, 1999, for the Game Boy / Game Boy Color and October 29, 2002, for the GBA, respectively). G&W Gallery 3 is easily obtained secondhand, or with some difficulty new, and G&W Gallery 4 is currently available on store shelves (The ROMs are rarish, due to its newness and Gameboy ROM crackdowns, but otherwise emulates perfectly.)
There was also a "Donkey Kong Jr. Math", actually released before Donkey Kong Jr. on the NES (an NES launch title, in October 1985), and it wasn't bad for edutainment, besides the fact that the controls are godawful. The cart for this game is rare, but not valuable, and the ROM is fairly scarce, due to the game's obscurity.
As for where Donkey Kong Jr. fits into the (admittedly loose) Donkey Kong/Mario Brothers universe, it has been implied by Cranky Kong's complaints in the Donkey Kong Country series that he is actually Donkey Kong Jr. Since Cranky Kong is DK's grandfather, this implies five generations of Kongs. (Donkey Kong, Donkey Kong Jr./Cranky Kong, an unnamed generation of Kongs, DK, and Diddy and Dixie's generation.) Donkey Kong Jr. has cameoed in the Game and Watch Donkey Kong, in the Super Gameboy version of Donkey Kong, in Super Smash Brothers Melee (as a trophy, of course), and the arcade version of Super Punch-Out. He is a playable character in Super Mario Kart, Mario Kart: Double Dash, and Mario Tennis.)
Sources: TheBooBooKitty, IGN, Classicgaming.com, Gamers.com, gamefaqs.com