Chain Chomps are obstacles in the Mario Kart series that may also serve as Items. They can either be chained to something and dive for opponents, or they can be set loose to wreak havoc. The Chain Chomp Item spurs racers through the track and knocks down anyone in the way. They express some dog-like behavior, notably barking.
Mario Kart 64
In Mario Kart 64, special tunneling Chain Chomps appear on Rainbow Road track where they travel at road level in the opposite direction of the racers in a regular zig-zag pattern. If touched, the driver will be sent flying up.
Mario Kart: Double Dash!!
In Mario Kart: Double Dash!!, Chain Chomps may be used as special items for Baby Mario and Baby Luigi, as well as Petey Piranha and King Boo, who may use any character's special item. It attaches itself to the front end of the kart and pulls the users forward for a few seconds, knocking over other racers in the way. Afterward, it detaches itself and runs off, following the course for a few seconds before ramming into the wall or going off-course. Chain Chomp was going to return in Mario Kart DS but instead was replaced by the Bullet Bill, a similar item. Bigger Chain Chomps also appear as obstacles on the Mario Circuit and Luigi Circuit tracks in Double Dash!!
Mario Kart DS
In Mario Kart DS, Chain Chomps appear in the original Luigi Circuit racetrack from Mario Kart: Double Dash!! and the new course Peach Gardens. The latter level has a lot of stray Chain Chomps, some with Item Boxes instead of chains. The Chain Chomp was going to appear as an item in this game and for the rest of the series, but the Bullet Bill replaced it as an item in the final version and its role is given to that item.
Mario Kart Wii
In Mario Kart Wii, a Chain Chomp appears on both the new and the Double Dash!! versions of Mario Circuit. Multiple Chain Chomps appear again in DS Peach Gardens. If a racer is touched by a Chain Chomp, he/she will roll over twice. There is an unused item roulette sprite named tt_item_wanwan.tpl inside Race.szs of a Chain Chomp that has a kanji for temporary (仮) beside it, meaning it was probably used for testing purposes.
After the glider section, a Chain Chomp guards the sharp left turn and pressures them into the overlooking chasm in the Wii U remake of GBA Cheese Land (DLC track). There are also two gigantic, loose Chain Chomps in N64 Rainbow Road that cause the track to shake (which allows for Jump Boosts). Any contact with them causes you to get damaged, obviously.