From the Super Mario Wiki
Klobbers are crocodile-like enemies that are part of the Kremling Krew. They first appear in Donkey Kong Country 2: Diddy's Kong Quest. These enemies wear barrels and attack by running into Diddy Kong and Dixie Kong, and can shove them into obstacles such as bottomless pits.
Donkey Kong Country 2: Diddy's Kong Quest/Land 2
Klobbers make their first appearance in Donkey Kong Country 2: Diddy's Kong Quest as common enemies. Klobbers hide in their barrels and wait for Diddy or Dixie. When the duo approaches, Klobbers attempt to charge into the Kongs with intent of shoving them into other enemies, into an abyss, or something equally as bad. The Kongs can knock a Klobber unconscious by bonking it on the head (either by jumping on it or throwing something at it). After a Klobber is knocked out, the Kongs can use the barrel as a weapon. Sometimes, Klobbers run off the edge of an abyss, just by chasing a Kong.
A yellow version of Klobber also exists. These yellow Klobbers use the same strategy as the green variety. The difference between the green and yellow Klobbers is that the yellow variety can make the Kongs drop their collected Banana Bunches. Also, gray Klobbers with red eyes exist as well. These Klobbers are the most dangerous, as whenever they hit the Kongs, they cause them to drop a Balloon. The majority of gray Klobbers are found in the Lost World. Red Klobbers (known as Kabooms) also make an appearance. However, they hide in TNT Barrels instead and explode whenever they make contact into any Kong.
The Klobber also appears in the Donkey Kong Country 2: Diddy's Kong Quest pseudo-sequel Donkey Kong Land 2. Here, they played the same role, and had the same attack patterns as the green variety.
Donkey Kong 64
Green Klobbers and Kabooms also appear in Donkey Kong 64. Here they were uncommon enemies, found only in Angry Aztec, Gloomy Galleon, and certain areas in Crystal Caves. This time, Klobbers damage the Kongs when they run into them. They are also immune to almost all of the Kongs' attacks. However, a shock wave attack or an orange grenade can take them out.
Donkey Konga 2
Klobbers also appear in Donkey Konga 2.
DK: Jungle Climber
Klobbers most recently appeared in DK: Jungle Climber, where they are uncommon enemies because only two appear in Spooky Woods. In this game, they climb on Pegs and they try to bounce into the Kongs. The area they appear in is very foggy, so the player has to look on the bottom screen map to see where they are climbing around.