In all versions of the game, there's an unused message that reads:

"Buzby sucks eggs, and Eastwood smells!"

According to the game's lead artist 'Jason Wilson', this is meant for one of the game's main programmers 'James Busby' as an inside joke. Later, it was revealed to be true by 'Chris Sorrell' in the RETRO Vol. 8 interview.
Contributed by ProtoSnake
According to MediEvil creator Chris Sorrell, he described MediEvil as a fusion of Capcom's Ghost n Goblins and Tim Burton's Nightmare Before Christmas. It had the working title 'Dead Man Dead' before being called MediEvil.

"The first design proposal for the game had the working title ‘Dead Man Dan’ and described a game that was a fusion of Capcom’s Ghouls’n Ghosts with the art style of Tim Burton – especially the look and feel of The Nightmare Before Christmas. In both cases these were things that I was a huge fan of back in the mid ‘90s. Lead artist Jason Wilson shared my interest in dark, gothic influenced artwork and we worked together to define the look and feel of the game."
Contributed by CLXcool
The Italian version of the game features different game over music from all other releases.
Contributed by KidDivinegon
There's an unused item in the game's data called "Lake Key", presumably intended to be used in The Lake level. It uses the same icon as the Skull Key, from The Hilltop Mausoleum level.
Contributed by ProtoSnake
When the game was released in Japan, there were several alterations to the gameplay and graphics. These largely consisted of slight changes to cutscenes and alterations to gameplay that made it more difficult.

The most noticeable change is to Sir Dan's character design. Both his in-game and FMV model were changed to have a golden helmet, matching the one that he wore when he was still alive. The helmet can be unequipped in-game, but Dan will always be wearing it in FMVs except for the final cutscene where it is blown off.
According to the game's lead designer, Jason Wilson, the helmet was added because of a fear that Japanese audiences would react badly to Dan's skeletal design, and that they would not see him as a hero.

His eye was also given an iris, whereas in the original it only had a black pupil. This change was retained in all versions of the game's sequel.
Contributed by EnoRed