Final sprites are on top, early/prototype sprites are underneath.
The player characters getting mad swole. Not sure what purpose it could be for.
Looking up; Sonic is the only one with animations for two directions. These are only found in the prototype's graphic data.
The prototype has the animations of Mighty and Ray appearing from the emblem, but they are not seen in-game; the final updates them with idle animations and adds highlights to their eyes. Sonic's head was redrawn for his finger-wagging animation, and Mighty has two copies of his title sprites, the latter (final) copy adding more highlights to his nose.
There are two copies of the character select sprites in the final graphics data. Sonic got a serious redraw to look less like a deflated balloon animal, and Ray's patagia were emphasised in the final. Mighty's copies are both identical.
And the characters' intro sprites are doubled too, mostly with minor shading changes. Sonic's arms are animated differently, Ray is resized and his and Mighty's faces were adjusted. Mighty is the only one whose early sprites also has the 'panicked' expression, which is different from the final's.
Eggman only appears in-person at the end, and all he does is run away, but these unused sprites suggest there was a more involved encounter featuring holograms, remote controls and a bomb-throwing Egg-o-Matic! The Egg-o-Matic and bomb (shown with a placeholder palette) are removed from the final, but the rest of the sprites can still be found in its data.
Some kind of wheelie robot eyeball thing. Estimated palette, only found in the prototype.
Only found in the prototype. Some kinda splashy orb-y sorta deal. Placeholder palette.
A debug menu, possibly for a level or graphic editor.
A spiky spinning disc and an extending rod with suction cups. Found only in the prototype.
The two pieces fit together perfectly, but for what purpose?
Only found in the prototype. A little drill-faced Badnik, I guess?
Assorted destructible machinery. Placeholder palette, only found in the prototype.
A big dome platform. Placeholder palette, only found in the prototype.
A robot fish, found only in the prototype. Probably intended for Wild Water Way. Palette is placeholder, natch.
"Eggman! What are you planning this time?!"
(translation by Plasma Captain)
Found only in the prototype. An exchange between Sonic and Eggman - Sonic never talks through word balloons in the final game, so its colour is otherwise unseen. This dialogue is grouped together in the tiles, though it might have been spread out across multiple scenes. Thanks to Plasma Captain for the translation!
Found only in the prototype. Only Eggman appears on the Game Over text.
I might be missing something, but I could never get these to display; one would assume it'd show when the dipswitches are set to multiple coins per one credit. As such, Mighty never shows in the attract mode HUD! His palette is estimated.
English word balloons. The game definitely had a limited release in English terrories, but not necessarily in English. SEGA clearly aspired to localise it, if this and the following graphics mean anything...
It's Robotnik from the Saturday morning Sonic cartoon! People first found out about this because of two sprites found in the background tiles, but there's full animations to replace everything Eggman has.
None of the unused sprites above are converted, so it was clearly based on the final release. The only graphics I can't find replacements for are the teeny tiny Eggman on the title screen, and Eggman riding the cage in the intro.
... which is shocking, frankly, as it goes so far to replace even the stage icons to incorporate his cone-shaped head. Now that's attention to detail!
If you open SEGA's arcade games in MAME and use the tile viewer, you're likely only given access to the background tiles, no sprites. You can open up the files in another tile viewer program...
... and you'll get a 4-pixel slice of a sprite if you're lucky. That ain't practical!
SEGA's arcade games slice their data into several files, and assumedly read them by interleaving their bytes. (is "byte-interleaving" the official term for this? is there an official term for this besides "needless busywork"?) For instance, SegaSonic the Hedgehog's first sprite bank is stored in:
It loads 2 bytes from the first file, then 2 bytes from the second file, and repeats. If you interleave 2 bytes from each file in the order above...
And you'll have properly assembled sprites on your hands!
Sweetheart pal Matt (who I can't thank enough for helping out and being moral support - i get worked up when i find unused stuff!!) made a program that interleaves 2-byte files, though just for convenience here's the merged graphics files in one handy zip.
Most of SEGA's other arcade games (and the prototype) use only 1 byte between files. nineko from Sonic Retro made a helpful little file to do that, The Interleaver (which Matt also provided a download for).
More trivia than anything: SegaSonic the Hedgehog's prototype only has one 8MB sprite bank, but the final has two, the latter of which is half the size and is a kind of 'addendum' - extra player animations, new objects, graphic touch-ups (like the title/intro sprites) or localisation gubbins. (all of SatAM Robotnik).