In the G1 cartoon, the Terrorcons have no real introduction episode. It is implied in one episode that the Terrorcons are on loan from the Quintessons and in others, just among Galvatron's forces.

In your head cannon, which do you prefer?

 

Given their animalistic nature and Hun-Gurrr's gluttony, I can see them being the Decepticon equivalent of Gnaw, in that they're loyal to whoever is currently feeding them! And given the Decepticons had their problems in Season 3 with Galvatron's insanity, Energon shortages, being exiled on Chaar, and what not, if the Quintessons supplied them with Energon for the time being, then they'll work for the Quintessons! But if the Decepticons give them something, they'll work for them!

 

I always liked the idea of them being Quintesson products, it's more interesting than yet another combiner team made by Vector Sigma or some other BS means. The "on loan" idea is really interesting and is why I like characters like Axor, Doubledealer, Swindle, etc, they bring a much-needed commerce angle to a war that's too often just about endless mythical McGuffins. Obviously it's implied the Quintessons built the Sharkticons, Alicons, etc but since they are supposed to have created the actual Transformers in the cartoon it was nice to see that sorta-happen during the show's events.

 

It does make sense that they'd have created animal-esque, low IQ transformers like the Terrorcons as well. Easy to control, unlike their original creations. It also adds more weight to the Decepticon/Quintesson "Alliance"; Galvatron turns to them to bolster his failed forces.

 

With the terrorcons treated in cannon as usually, incompetent, quirky, and uncivilized, I always imaged their origin as the decepticons scraping the bottom of the barrel for muscle in the wake of unicron. I mean, they've already used their army corps of engineers, special forces, street gangs, and bounty hunters. These idiots were the compatible only group of five left.

 

i like both ideas.
i like hun gurr and co., but i like the idea of a quintesson army too. maybe hun gurr and the others arent terrorcons, just decepticon savages.

 

In my headcannon they are Decepticons, but having them on loan from the Quintessons works for me in the cartoon.
You could also interpret that episode where they worked with the Quintessons as the Decepticons working as henchmen of the Quintessons from time to time for financial reasons.

 

In my scrambled eggs-for-brains head canon, they and the Predacons were both built by the Quints and gifted to Galvatron in efforts to solidify their alliance -- or so they wanted Galvatron to believe.

The Predacons came first, of course. Some Quintessons were skeptical about turning over five mighty warriors that turned into a tactical nightmare like Predaking, but the higher-ups assuaged their concerns: give Galvatron some new toys (including helping them build Trypticon), and maybe the madman will fight for them until they could flip that stupid "off switch," at which point Predaking, Trypticon and any other new guys would pose no threat.

Obviously, that didn't work out as planned   However, sometime later, the Quints -- perhaps a different faction -- tried to cozy up to the Decepticons again, this time with the offer of the Terrorcons. But hidden in their programming were homing beacons and that time limit on how long they could remain Abominus, so the Quintessons could summon HunGrr et al. when they needed serious muscle but didn't want to get their own tentacles dirty. After "Money is Everything," Galvatron found out about this deception and had the Terrorcons' programming wiped of any five-faced Slime of all nebulae b.s.

It's not perfect, but that's my take