Transformers: Beast Wars Transmetals 64

Ravage and Tigatron:
Select vs. or arcade mode. Highlight Cheetor, then hold Z and press A at the character selection screen. Then, press Left or Right to go through his colors to get to Ravage and Tigatron.

Starscream:
Select vs. or arcade mode. Highlight Waspinator, then hold Z and press A at the character selection screen. Then, press Left or Right to go through his colors to get to Starscream.
Information in this section was contributed by Randall Blomberg and kopaka95.

Black Arachnia:
Select vs. or arcade mode. Highlight Tarantulas, then hold Z and press A at the character selection screen. Then, press Left or Right to go through his colors to get to Black Arachnia.

Original Rattrap:
Successfully complete the game, including defeating Megatron X. Select vs. or arcade mode. Highlight Rattrap, then hold Z and press A at the character selection screen. Then, press Left or Right to go through his colors to get to his original appearance.
Information in this section was contributed by Keith123088.

Alternate costumes:
Select one of the following characters and press Left or Right. Optimus, Trantulas, Airazor, and other characters have a strange Blue form.
Information in this section was contributed by Keith123088 and Louie.

Airazor: TigerHawk look-alike
Airazor: Noctorro Look-alike
Cheetor: Young Tigertron look-alike
Megatron: Transmetal 2 Dinobot look-alike
Opimus Primal: Him with Predicon symbol (in beast mode)
Optimus Primal: Optimal Optimus
Optimus Primal: Polar Claw Look-alike
Rattrap: Odd Rattrap
Tarantulas: Cyborg Tarantulas
Tarantulas: Old Tarantulas
Terrorsuar: Old Terrorsuar (colors)
Terrorsuar: Depth Charge Look-alike
Terrorsuar: Onyx Primal Look-alike
Waspinator: Buzz Saw Look-alike
Waspinator: Insecticon look-alike

Fight Megatron X:
Successfully complete the game with Megatron with no time limit. You will have to fight Megatron X at the end.
Information in this section was contributed by Lenur3.

Defeat Megatron X:
Press C-Right to keep firing your gun until you get a Energon Strike. Transform into vehicle mode. Then transform into beast mode and keep pressing C-Right rapidly until your Energon bar is filled. Get close to Megatron X. Repeat this you defeat Megatron x twice.

Another advantage happens when both of you shoot something at the same time, after you fire block. Your shot should hit and his should do minimum damage.

Metal Megatron:
Defeat Metal Megatron in arcade mode to unlock him as a playable character.
Information in this section was contributed by Dirtdude51.

Hint: Easy win:
For an easy win, transform as in vehicle mode. Run/fly around the arena and keep pressing C-Left or C-Right to use good weapons. Then, when your enemy is half way down in HP, turn into robot mode, hold C-Left for about fifteen seconds, and release. Your character should do a strong blast and should defeat your enemy easily.
Information in this section was contributed by Dark Cloud.

Glitch: Robot watcher:
Play on the Ice stage, and look at the background closely. You should see a robot that is watching you. Note: This does not happen very often.
Information in this section was contributed by Dark Cloud.