Pokémon Pinball is pinball with Pokémon. Go figure.
A 16×16 block appearing as-is among the game's tiles, part of what may have either been a table or a simple image that wasn't used in the final. If this was going to be a table, then the center area would have likely added an object to send the ball back out or a hole for it to fall into.
Unused Pocket Monster Pictures
These two pictures only differ by a few pixels. They were likely meant to be used in the space where map pictures or wild Pokémon pictures appeared in the lower half of the Red and Blue fields.
Unused Japanese Font
Unused Debug Menu Font
A font used by the inaccessible debug menu described below.
Unused Debug Menu
A normally inaccessible debug menu is present in the game's code. It allows the user to run the game as either Game Boy Color or the DMG model. Since Pokémon Pinball is supported on multiple Game Boy platforms, the developers probably used this debug menu as an easy way of testing without switching physical Game Boy models. This menu was accessible by holding the Up button when turning on the game.
To enable this menu, change the byte at ROM address 0x2021 from $01 to $00. Then, hold down Up while turning on the game.
The European version prompts you to select a language the first time the game is powered on. This screen cannot be accessed again unless SRAM is erased, though the language can always be changed later via the Options menu.
The US and European versions have a couple more companies incorporated than the Japanese version. The Japanese version's copyrights also have a little more space. The European copyright screen was altered to account for its later release date.
The European version changed "Poké Dex" into the slightly more correct "POKéDEX". Additionally, Pikachu was given a smile in the international versions.
Besides the ability to change language, the European version also allows you to choose between "Off", "Mild", and "Strong" rumble settings, unlike the US and Japanese versions, which only have "On" and "Off". Also, the "M" in "RUMBLE" is slightly wider in the European version.
The US and Japanese versions allow you to freely configure the control scheme. This was probably considered too confusing and was simplified into three predefined control schemes for the European version.
Super Game Boy Border
The European Super Game Boy border reuses the cartoony "Pokémon Pinball" logo from the title screen, while the Japanese and US versions use (arguably more stylish) lowercase sans-serif text. Additionally, the Japanese border has no trademark (™) symbol.