There was a model for it, and an animation that showed it barely fitting onto a large pad. It was meant to be the largest ship to squeeze in through the mailbox, a third to a half longer than the Type 9/10. IIRC the Devs actually said during Focused Feedback that it was shelved indefinitely because they didn't have a balanced role for it.
They might have named the Fleet Carriers the Panther Clipper, but I really really doubt it. It wouldn't be a good fit. The Panther was always a pilotable trading ship.
And I don't think the concept art of the large ship launching smaller ones, that people are now associating with the Panther, was ever named or connected to the Panther Clipper by Frontier (could be wrong here), and it doesn't actually resemble the concept art of the Clipper that was shown.
The only connection the Panther Clipper has to Fleet Carriers is that they're both big. That's it. And not even the same kind of big.
I'm so sure the Fleet Carrier is not the Panther Clipper that I promise I will self-destruct my Krait Phantom out in the Trojan Belt with three months of exploration data if Frontier announce the Fleet Carriers and Panther Clipper to be the same thing.