Bug Type

Bug type attacks are super effective against psychic and dark types. Many psychic and dark types have low defense stats which means that these bug type Pokemon might just have a one-hit KO against psychic and dark types!

Scyther is a bug and flying type. Slash is an obvious strong move to go with and will evolve into a steel and bug type. Scyther's attack is decent and so is its speed. The only setback is that Scyther can only be found in the Safari Zone, which makes it hard to catch(Except for Pokemon Gold/Silver where you can find it in the park above Goldenrod City).

Pinsir is a bug type that has excellent speed (and for all strong trainers, it also carries swords dance which will make battling easier). Similar to Scyther, Pinsir can only be found in the Safari Zone.

Yanma is a single evolution Pokemon(until the fourth generation) and has good speed. It is a bug and flying type and can be found in Pokemon FireRed and LeafGreen on the island where there are ancient ruins(I think it's the island where Dotted Hole is situated at and that island is the island where your rival(Gary) is in the Pokemon Center)

Heracross is a great Pokemon who is a Fighting and Bug type. I think that it's an awesome Pokemon with only two main weaknesses which are the flying type and the psychic type. Megahorn and Earthquake are two moves you should definitely equipt Heracross with. You can find Heracross in Pokemon FireRed and LeafGreen on one of the islands(I'm guessing Six Island).

Volbeat is a Pokemon who can be found in Pokemon Ruby/Sapphire/Emerald and is a single evolution Pokemon as are the others on this page. Volbeat notably gives you higher EXP points than the other Pokemon(except for Illumise) in its area, which is the grass to the south of the daycare(which is to the left of Mauville City). One of its signature moves is Signal Beam which is pretty strong.

Illumise is similar to Volbeat so I won't go into detail.