It might not surprise you to learn that I am, in fact, a big fan of the Pretender gimmick that Hasbro employed for the Transformers brand in 1988. I think it's just great. Part of the reason for this is that I like the idea that the Transformers are technologically advanced enough to use organic material as a further disguise for themselves (something that would be built upon even more in 1996's Beast Wars). To be perfectly fair though, we in the west never really got to grips with the idea, and what we ended up with was 20ft tall humans, which completely went against the Pretender idea in the first place. The Japanese understood though, so you had Transformers that were human-sized, mass-shifting all over the place. Much more logical, to my mind anyway.
The Decepticons got off a bit lighter than the Autobots, seeing as they all had monster visages, I'm much more willing to accept a 20ft fish monster than I am a human. Such is the case with Finback, who DOES transform into a 20ft fish monster. Well, his outer shell was, anyway. His inside was a generic looking robot who transformed into a hovercraft BECAUSE HASBRO SAYS HE DOES SO FUCK YOU THAT'S WHY. His whole backstory was that he has a debilitating disease, the result of a raid on a world dreadfully polluted world (some environmental message perhaps?) and so his shell is his refuge, a place where he can be free from the ravages of his condition. I always loved that justification, it gives the shell some actual purpose rather than 'it's a disguise'.
As for fiction, well, Finback got a little bit, but nothing too meaty I'm afraid. He was with the other Decepticon Pretenders when they fought Underbase Starscream (his attacks doing little to damage them), he was part of the team that created Scorponok's massive underground base in New Jersey, and then he fought against Unicron. Everything was going pretty well for him until the power core of the mobile turret he was firing blew up, killing him and Misfire in the process. It's always the way.
In the IDWverse, he was recruited by Bludgeon to undergo the Pretender process started by Thunderwing, and was doing ok, grafting more bio-mechanical carapaces onto himself when Jetfire and the Autobot survey team stumbled upon his team. Sadly, Finback's torture of Jetfire was cut shot when he was blasted apart by the Wreckers.
As I said before, Finback's Pretender shell is a fish monster thingy. Really, that's the best description for it. The only articulation it has is in the arms, mainly due to the fact that it has to split apart to accommodate the robot component, which again, is limited in it's articulation. The only bits that has really is to form the basis for the transformation, into it's hovercraft mode. If indeed that's what that's supposed to be. Have I mentioned yet that I love Pretenders?
All in all, Finback is a toy that you're either going to really love or despise with all your being. I love it, I know plenty who don't. If you're new to the world of Pretenders, or you're on the fence, Finback's a good place to start. He has a freaky Pretender shell, and his inner robot isn't too shabby either (especially for a Pretender).