The Starscream
A Big Bad's treacherous right-hand man/woman.
When a villain have henchmen, sometimes one of them could be more ambitious than the others (sometimes, could be more than one) and want to overthrow the boss to be in charge.
The difference between the Dragon and the Starscream are meanwhile the first one is loyal and trustworthy to the Big Bad, the second one is treacherous and disloyal.
Examples:
Transformers: Starscream is practically the trope namer, every incarnation of this cybertronian was Megatron's right-hand man, but he want to be more than that and he tries to overthrow him since '84. If you want other examples in the franchise; Shockwave from Transformers Marvel Comics, Airachnid from Transformers Prime, Terrorsaur, Tarantulas and Blackarachnia from Transformers Beast Wars.
Doctor Who: Since he appers in the episode "Genesis of the Daleks", Davros was betrayed by his own creations, the Daleks. Since then, the Daleks have repeatedly turned against and overthrown their creator, only to come crawling back when they are weak, because he is smarter than them. Unfortunally, in "The Stolen Earth", he Daleks don't even pretend to respect him this time, and are keeping him as a "pet".
Henry Stickmin: Reginald Copperbottom was the original Toppat Clan leader, but Henry become the new leader. In "The Betrayed", Reginald doesn't rescue Henry and leave him for it's own. In fact, in his biography, Reginald did the same move to the former Toppat Clan leader, Terrence Suave.
Bionacle: Roddaka is this to everyone who's working for. When Sidorak ordered her to fight Keelongu, she leaves the battlefield. The results; Sidorak is killed by Keelongu, meanwhile Roddaka is safe and in one piece.
Crash Bandicoot It's About Time: When Uka Uka opens an interdimentional rift outside the primitive times, he's tired by the force he needed to make it. Then, N. Tropy and Neo Cortex leave him there, saying he served his porpuse.
Super Paper Mario: Dimentio succeeds through manipulating everyone to use the Pure Hearts to defeat Count Bleck so he could control the Chaos Heart and destroy all worlds and create perfect new ones so he could be a god.
Dragon Ball Z: Vegeta was essentially this to Frieza, serving him until he discovers the Dragon Balls and planning to became immortal and overthrow him. Frieza was aware of this, but was so far out of Vegeta's league that he never considered him a threat and that when he did turn on him he could kill him easily.
Pirates of the Caribbean: Hector Barbossa was this for Jack Sparrow. Originally, Barbossa was Sparrow's first mate, until one day he and several other pirates on Sparrow's ship decided to get rid of their captain by throwing Sparrow overboard once Sparrow gave up the bearings to their treasure, and as a result Barbossa becomes their captain instead.
