Comic creators pitch new characters and stories all the time, and naturally some get rejected or dropped mid-production by the company they were originally offered to. Equally naturally, some creators then go on to reuse the proposed ideas, perhaps with some modification/evolution, somewhere else. And that's what this thread is for - identifying characters who were originally planned for one company/comic continuity, but who ended up actually being published elsewhere.

Starting us off:
Circa 1989 then rising star Rob Liefeld pitched a Young Avengers series. Most of the proposed members were existing characters, but two were to be new. Unfortunately, although the pitch was posted on Tom Brevoort's Marvel Blog several years back, it's long since vanished off the internet, with only one page to be found on the Wayback Machine, so most of the details are unavailable, but Liefeld himself did confirm that one of the new guys was intended to Brahma

who went on to be a Youngblood character


Meanwhile Rob Liefeld also planned to introduce a character called Cougar to the New Mutants, as revealed in Marvel Age:


Though technically he did actually appear in-universe in a crowd scene in Avengers West Coast, I think we can still count him, and he too went on to become one of Youngblood:


Then there was Phil Hester's Young Avengers, that was going to include the humanoid son of Fin Fang Foom. No pictures of the design sketches this time, sadly, but when Marvel knocked that back, the son was moved to Image to become Firebreather.

who is the son of a dragon who doesn't resemble Fin Fang Foom at all. I mean, here's Fin Fang Foom alongside Firebreather's dad. 
Obviously they look completely different.

Next up, Paul Grist's Jack Staff. He's slightly different from the above as he was based on an already existing Marvel character, rather than being a new one, but the outcome remains the same. When Paul's pitch for a Union Jack miniseries was rejected, Paul reworked it for the new character of Jack Staff:

Admittedly, anyone whose costume is based off a real world flag is always going to bear some resemblance to another character whose costume is based off the same flag, but even allowing for this, the source for Jack would still be clear even if Paul hadn't been quite open about how the character came about, not least because Jack's first story included other characters who were clearly analogues of Captain America and Baron Blood.

Then there's Larry Hama's Fury Force, intended as a special ops group of SHIELD agents:

who morphed into GI Joe:

I only learned today that Cobra Commander was originally Baron Strucker, and that Cobra were Hydra!

Lastly, for the moment anyway, there's Nightcrawler, who was originally conceived by Dave Cockrum to be an alien and member of the Outsiders in the Legion of Super-Heroes' 30th century:


And that's all the ones I can think of off the top of my head. However, I'm sure there are more. Feel free to add to the list.