Transformers: Beast Wars Transmetals 64 

Play as Ravage and Tigatron

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

Play as Starscream 
Select versus or arcade mode. Highlight Waspinator, then hold Z and press A at the character selection screen.
Press Left or Right to go through his colors to get to Starscream.

Play as Blackarachnia 
Select versus or arcade mode. Highlight Tarantulas, then hold Z and press A at the character selection screen.
Press Left or Right to go through his colors to get to Blackarachnia.

Play as Megatron X 
Defeat Megatron X in arcade mode without using any continues to unlock him as a playable character. 

Fight Megatron X 
Successfully complete the game with Megatron with no time limit.
You will have to fight Megatron X at the end.

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.
keep pressing C-Right rapidly until your Energon bar is filled. Get close to Megatron X.
Repeat this until you defeat Megatron X twice.
Get Rattrap's armor from the first episodes Successfully complete the game,
including Megatron X to unlock his best armor.