@Chainspike: No, according to TCRF, there is another scan of the same image in better quality, revealing it was meant to be day in Act 2. Also the Sonic 3 prototype.

This could indeed be the original palette, an early screenshot actually shows Act 2 using the night palette as seen in Act 1 (perhaps it would have transitioned to day after the act 2 boss?). http://info.sonicretro.org/File:Sonic3_MD_Development_ICZ_01.jpg

In a comparable situation, this site seemingly has the "on fire" Angel Island only in the "Act 2" rip, despite first occurring in Act 1. Granted, I just woke up and am basing that off the icons (with one being a .zip), so....

Why not just Ice Cap Zone Act 1 Boss?

@YGbJm - Yes, maybe and no.

Yes - Night is correct, but ICZ Act 1 was originally meant to be at night.

Maybe - The transition to the Daytime palette still occurs in Act 1. It'd make sense to rename it but at the same time, it would still would work out the same way without the renaming. Everyone who's played the boss will recognise this as the palette used up until Big Icedus explodes.

No - This is the first palette, but since the Act 2 palette is on-screen longer, that's more commonly cited to be the actual palette. They're both correct, as well as the eight between them during the transition.

I feel like this should be renamed to Ice Cap Zone (Outdoors, Big Icedus/Night) since it's more specific and also since it's literally act 2's outdoors except recolored.