This is something that went from a passing joke to actually being just a touch concerning as a possible reality. Personally, it still seems impossible to me, but as people have really slowed the image down and researched it a bit it doesn't seem entirely impossible.
For those who haven't seen it, Nintendo recently released a trailer for the coming Sonic games in their recent Nintendo Direct, and a new secret ally was briefly shown as little more than a silhouette seen here:
https://youtu.be/Z7OUOQjz_pM?t=29
For reference, this is Bubsy:
Now, there's something funny about that quick reveal... upon seeing it, I immediately went to the comments to joke "I think it might be Bubsy", only to find apparently everyone else saw the same thing. That everyone was seeing Bubsy seemed quite odd, which prompted people to really investigate this possibility.
There's two main differences that can easily be explained away in that he's wearing gloves and he's wearing shoes/socks, but that would be a mandatory adjustment in a Sonic universe where that is true for everyone. Otherwise, they seem to match up frighteningly perfectly.
Now people who know far more about the Sonic Universe than I have been brainstorming over who it could possibly be, and as of this moment there's no Sonic characters who have appeared in a game that match this character, and even in the extended universe none really match it.
So far, all we know is that Sonic Boom characters aren't appearing, and that he/she is "an unexpected character" that "plays an important role in the story, and embodies the overall features of the game".
Otherwise, Bubsy is currently owned by Tommo Inc., someone Sega has worked with in the past and Tommo currently has a license to sell cell phone cases with the likeness of Sega characters on it, so the two are very familiar with one another to this day.
Still, despite all of that I feel this just can't possibly be true... The idea that somewhere Sega people were sitting around brainstorming about how to rejuvenate the Sonic brand and someone introduced the idea to include one of history's most generic and annoying mascots and have him play an "important role in the story" is just a little too difficult to accept.
That said, if this were some popular character that had been absent for ages the current evidence would easily put me on the "it's probably him" side of the equation, so it's really only the absurdity of bringing back such a silly character that's maintaining my overwhelming skepticism. Still, thus far they've described him/her as a character that's unexpected (as in already known but not seen in a long time, if ever in a Sonic game), he doesn't match any known Sonic characters, and yet it seems everyone's gut reaction upon seeing him is that it is clearly Bubsy...
Here's hoping it's somehow someone else, as I can't imagine the trolling Sonic videos would get if it's him lol
These games look promising, there's no need to introduce an onslaught of terrible cat puns to this equation :p