From the Super Mario Wiki
Beetles are insects that have made a few appearances throughout the Mario series. Enemies found in Mario games called Buzzy Beetles, Parabeetles and Buster Beetles don't resemble normal beetles, despite their name, resembling more as Koopas instead.
 Donkey Kong 3
Beetles make their first appearance as enemies in Donkey Kong 3. They appear in the 18th round and go down to the bottom of the screen, just like Buttiflies, and then charge across full speed, attempting to hit Stanley the Bugman.
 Super Mario Land 2: 6 Golden Coins
 The Super Mario Bros. Super Show!
Beetles appear in The Super Mario Bros. Super Show! episode "Princess, I Shrunk the Mario Brothers", where they have a rather minor role. A quartet of Beetles are shown to be living inside a drainpipe that a shrunken Mario and Luigi take refuge inside of. As the Beetles are hungry, they chase Mario and Luigi through the drainpipe in an attempt to eat them, although Mario and Luigi manage to escape. In this appearance, the four Beetles resemble a cross between an insect and The Beatles.
 Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Booster collects beetles, but does not catch them himself, instead forcing his Snifits to make a hobby of capturing the beetles for him. By renting a Beetle Box for 150 coins and traveling to Booster Hill, home of the Beetle Race minigame, anyone can join in the fun and try to catch beetles along with the Snifits. Each Beetle can then be traded at Beetles Are Us for a predetermined amount of coins. Golden Beetles are worth one Frog Coin, male Beetles are worth fifty coins, and females only one coin. A Beetle also appears as a playable character in the Beetle Mania game that can be purchased from the Beetle Mania Toad in the Mushroom Kingdom for 500 Coins.
 Luigi's Mansion: Dark Moon
Beetles appear again in Luigi's Mansion: Dark Moon as an ambience enemy that appears in and after the Old Clockworks, this time, beetles are small green or black creatures that can hurt Luigi if he gets close enough to one. Gold Beetles are also found, which drop one Gold Bar. Beetles in Luigi's Mansion: Dark Moon have a spike, two small wings and are green in the Old Clockworks and black on the Secret Mine.