There are two variants of Beetles: black beetles, which are the weakest and more common type and red beetles, more powerful but less common, but even the latter can only pose a threat in a large swarm, which is very rare.
Beetles are the first enemies faced by the Hero of Oakvale. They can be first encountered in some barrels inside a few warehouses in Oakvale, when the Hero is tasked with guarding man's stock for a gold piece, if he chooses to destroy them. They are also encountered in the Guild Woods, during the Hero apprenticeship at the Heroes' Guild. They also appear both in Greatwood, especially if the Hero is at a low level, and in the Temple of Avo, while doing the quest The Ransom Victim.
In Fable II, beetles are also the first enemy encountered by the Hero of Bowerstone (not counting the bully Rex). They are first encountered in the upper floor of Balthazar's warehouse, when the Hero is asked to get rid of them. These beetles however are harmless and will not attack the Hero. They can also be found both in the Guild Cave and Bower Lake. The beetles here will attack upon detection but do not pose much of a threat.
In Fable II and its DLC, beetles can jump onto an NPC's face, doing a small but constant flow of damage. The amount of damage the beetle does depends on what type of beetle it is. The NPC will probably be able to throw it off, though, the quicker they do so will depend on the NPC's physique, sometimes the victim will die if they are already hurt or very weak, but it is rare. There is also a unique death sequence that plays when killed; They will fall to one knee, and then backwards onto the floor, dead, as the beetle climbs off the victims face. Note that this can also happen to the Hero, but it is rather rare, and instead of depending on the characters physique the player must instead press a series of buttons. When a beetle if thrown onto the floor, it takes a large amount of damage and that alone can be enough to take out the weaker and/or injured ones.
In Fable III, early on the creatures summoned by the summon creatures potion appear to be some form of beetle. This is their only appearance in the game, although industrial-age robot beetles make an appearance in the Traitor's Keep DLC on Clockwork Island.
In Fable, Fable: The Lost Chapter and Fable Anniversary, if a guard is nearby the beetles inside smashed barrels in the Birthday Gift quest, the guards will attack them.