Apologies if offence was caused; it's just that as a layperson I find creator rights a little perplexing at times (e.g. can someone please explain why Dave Gibbons gets the credit for Harlem Heroes instead of Carlos Trigo?)
Unless some special arrangement is made, it's usually as simple as whoever's name is on the first episode published — you don't get a creator credit for having been involved in an unused/abandoned pre-publication version. Technically, Robo-Hunter should be credited to Ferrer/Gibson as joint artistic creators, since both artists' work feature in the early episodes… I don't have a reprint volume to hand to check whether that's the case.
If those initial Ferrer episodes had been scrapped entirely, he would't get a credit. McMahon gets the artist creator credit for VCs, Angie Kincaid for Slaine, despite only having drawn one episode of each, because it was the first.*
The only big exception I can think of is Ezquerra's creator credit on Dredd, but that's an unusual one since his episode was the first drawn, and then handed off to McMahon, Gibson, et al, to work from his designs, but ended up not being the first published.
In some respects, I think Trigo got a bit of a rough deal on Harlem Heroes, though, since a fair chunk of his design work for the Prog 0 dummy seems to have survived into the published Gibbons version in Prog 1… although I'm not sure we have any way of knowing whether he was given a design brief by someone else (Doug Church, maybe?).
*As I've mentioned before, it's hard to argue that any modern incarnation of Wolverine owes more to the original Wein/Trimpe guest star in Incredible Hulk 181 than it does to the Claremont/Cockrum/Byrne version from Uncanny X-Men, but Wein and Trimpe get the credit because they
created the character. Re-defining the character, no matter how well, or how definitively, doesn't get you squat.
The only exception to that I'm aware of is Jamie Delano's co-creator credit with Alan Moore for John Constantine, but that only happened because Moore asked for it to be that way.