I should let you guys know that Hasbro(the company that runs Transformers) has recycle character names. You can see a character in the Unicron Trilogy or Animated that has the exact name as a G1 character but there is barely a resemblance to the G1 character. It doesn't mean they're a re-imagined character, they're simple a name that Hasbro has a copyright to and recycle the name. This was common before and after the TF movies. Half the cast in the movies were simply original characters whose name was reused from an earlier source.
Here's the 2007 Decepticons
This is Starscream, Megatron's 2nd-in-command. Starscream wants one thing, leadership of the Decepticons. In most versions of Transformers, he finds whatever opportunity to overthrow Megatron. It was downright laughable in the original show since he did it to Megatron pretty much every episode but it seems Megatron forgives him and he's ok in the next episode like nothing happens. Later shows gotten better with it, he either had to really really did something to get back at his good graces or kicked off the Decepticons for a season.
Sadly, the Starscream's betrayal story never happened in any of the movies besides a line spoken by Megatron that Starscream was in no hurry to get his leader revived in the 2nd one. In The Last Knight, Megatron held Starscream's dead head and called him his traitorous friend, which is strange, due to Starscream never did backstab him at all in the movies.
Barricade is the bad police car. There was a G1 version of Barricade but it was introduced late in it's run when the cartoon stop running while the comics and toys were going. G1 Barricade was very forgettable. The video game, War For Cybertron did have Barricade as one of the playable Decepticons, his personality was like his G1 counterpart but his design and colors were based on the movie version.
Barricade has an interesting history of disappearing. At the climax of the 1st movie, the Decepticons were doing a rollcall scene as they were heading into the city to fight the Autobots, which included Barricade. During the entire final battle, he wasn't seen anywhere. People say his survival may be a story for the sequel, which never did happen.
After four long years of being missing, he finally showed up in the 3rd movie as the Decepticons were gathering Autobots to execute them one by one. Suddenly the human military comes to save the Autobots. The military shot out Barricade's eyes, blown off his legs, and rain down ammo on him. He's dead alright. Then all of sudden, he's in The Last Knight with no explanation of how he survived.
Blackout, the evil helicopter that blew up the military base was based on nobody. There was a toy during the late years of G1 but that's it. A character in Animated as an homage from the movie did appear.
Frenzy, the evil sneaky bot that sneak around is based on one of Soundwave's cassettes.
Brawl the giant tank bot they face at the end is based on one of the Combaticons. A group based on military vehicles that combine into one big bot.
Due to an error, Brawl was referred to as Devastator during the Decepticon roll call scene before the climax. It was an early production name for him and he was accidentally called that instead of his final name. The actual Devastator was showed in the 2nd movie(with wrecking balls. Uck!)
Scorponok, the giant metal scorpion that hunted the military in the desert is loosely based on several characters over the years with the same name. In G1, he is a evil organic citizen of the planet Nebulos who turned himself into a transforming head of the mighty fortress of Scorpinok. He's sometimes the main leader of the Decepticons at times when Megatron/Galvatron are not around.
There are other versions over the years that looked different in designs and personality. In Beast Wars, he's Megatron's dumb loyal bot. In Energon, he's a clone made of memories of a planet destroyed in the past.
This skater bot is Bonecrusher. He's name came from one of the Constructicons. However in the movies, he is not one of them.
There is a popular meme called "Bonecrusher hates X". It's because his bio says he hates everything.
Fun Fact about the 2007 movie: Blackout, Barricade, and Frenzy were all all intended to be Soundwave originally. He was a main dude in the Decepticons in G1. The problem is that the writers didn't like how their "Soundwave" was ending up and rewrote them to original characters. Think about it, when Blackout was laying waste to the military base, he ejected Scorpinok out of his chest(who was originally meant to be Ravage). Frenzy turn into a boombox and playing espionage on Air Force One. Also, Barricade had Frenzy coming in and out of his hood. It was finally in the 2nd movie that Soundwave made his appearance.
Now for the Decepticons introduced in Revenge of the Fallen.
Grindor(The other Helicopter that Optimus ripped apart) is an original character. He's pretty much a brown version of Blackout.
This is Alice, the transformer disguised a human. Believe or not, there were Transformers that transformed into humans back in the late 80s called Pretenders. This was after the cartoon was cancelled but the toys and comics were still going. However in the comics, the Autobots tried to blend in as 25-tall humans while the Decepticons tried to blend in as 25-tall monsters. Yeah, that's not going to work.
Ravage is one of Soundwave's cassettes and does spying missions.
Scalpel, the bot that try to get into Sam's head is based on nobody.
Demolishor, the giant unicycle bot that easily got defeated in the opening is based on no one really. There was a character from Armada with the same name but other than the name, nothing more.
There been alot combiners over the years but if there one that everyone remembers, it's Devastator. Several construction vehicles merging into a giant colossus.
This Robo Bug you briefly see is called an Insecticon. They are based on a group of far larger bugs. They multiply and devours everything.
Sideways, the car that got ripped in half is a original character...or not. Like The Fallen, he may or not may be a pre-existing character that exist throughout the multiverse. Yes, this causes headaches.
This convoluted character 1st appeared in Armada, he seems like a good guy but betrays them and work for the Cons, then he betray them and it was revealed he was working for Unicron. He said he was Unicron's avatar but Armada had a very terrible rushed dub and no one took it seriously(and the Japanese version didn't get fansubbed till a decade later) Others say he was created by Unicron.
In Transformers:Cybertron, he's back and with a different backstory, saying he's a survivor of Planet X. Cybertron was meant to be a stand alone show in Japan but America retroactively made the show a sequel to Armada and Energon. A bio was posted online that explains that Sideways' backstory was that he made a deal with Unicron after his Planet was gone.
Allspark Almanac which is a guide to the universe of Transformers: Animated had a brief entry about a character named Sideways and he soon felled in a portal. The entry joked that he always wanted to visit China. Possibly hinting he's the same dude in the movie. The author of the book said it was intended to be a joke and not to be taken seriously. It also teased it's the same exact Sideways from Armada
A facebook account was made to answer questions in 2015 and Sideways took over and acted like a troll. He said alot of things that was obliviously false. He mentions that every Sideways is the same. That all his convoluted different origins is all true...or not.
All the bios on the actual movie Sideways said he was a coward. Which is noting like the sneaking backstabbing troll.
Now for the Decepticons introduced in Dark of the Moon
Laserbeak was one of Soundwave's cassettes and he was sent out to do alot of air attacks in G1.
The Dreads are a trio of bots that chase after Sam and Bumblebee down the road and later confronted the Autobots at a Mexican standoff. Their designs were influenced by the famous movie alien, Predator.
Now for Age of Extinction
Stinger is one of the human-built Transformers. He's meant to replace Bumblebee, meaning he's an Anti-Bumblebee.
Steeljaw are one of Lockdown's hounds. There been several animal-like Decepticons with that name.
Now with The Last Knight
Megatron had to bargain with the human army for his troops to be released, they have loud and boastful personalities, have Suicide Squad-type text introductions...and they die not long after.
This loud-mouth, chain-wearing tattoo jerk, named Nitro Zeus is based on nobody. He's is a sort of redesigned model of Shockwave. Notice he has one eye.
This fast-talking motorcycle punk is called Mohawk and he's based on nobody.
Onslaught is a sort of redesigned model of one of the Constructicons.
Dreadbot is a sort of redesigned model of one of the Dreads.
Berserker never did got released from his capture. Despite the lack of screentime, he somehow had more toys than the rest who had a little more screentime and lines. He's is a sort of redesigned model of one of the Dreads.
Infernocus is a combination of several monster like Transformers sent by Quintessa to help Megatron. He may or may not be loosely based on Abominus, a group of monsters combining.
