Castlevania: Nocturne in the Moonlight
Ports of games are common enough -- the original Castlevania had several ports to the various computer systems. With the rousing success of Symphony of the Night, Konami took the opportunity to port the game from the PSX to another contemporary system at the time (that was popular in Japan), the Sega Saturn (this game is refered to, by American fans as, Nocturne in the Moonlight, which was the original Japanese name for the original Symphony, but it does help to differentiate the two tiles for American audiences).
For various reasons, though, Nocturne in the Moonlight was a flawed port. The Saturn was not a compatible piece of hardware with the PSX, so compromises had to be made for the game. Transparencies, for one, were not something the Saturn could reproduce, so various transparent effects from the original game were done with dithered sprites (transparent "checkerboard" effects that end up looking pixelated). Some items that used transparency, such as the Invisibility Cloak, were replaced with other, functionally similar items that lacks the transparent effect entirely.
Worse, though, was simply that the Saturn was not well-designed for 3D graphical effects. Although the PSX Symphony was a 2D game, it used the 3D engine of the underlying system to create many graphical effects. These effects, for whatever reason, were kept, but they cause severe slowdown whenever on-screen (included in the save rooms, one place you wouldn't expect to find slowdown). This was coupled with much longer load times for areas (and menus, and maps), which slowed down the entire pace fo the game.
The aspect ratios for the two games were different, too. To compensate, the sprites were compressed, and then stretched to comnpensate for the different vertical and horizontal dimensions, leading the sprites to look fuzzier, more aliased, less crisp.
For all of these reason, the general opinion (among reviewers and fans) was that the game was a hobbled port of the original game. The Saturn was just not capable of being a PSX, and it showed. Being the reviled port of a fantastic game pretty well meant that American audiences had no chance to play this version of Symphony. Additionally, while the Saturn was moderately weel recieved in Japan, it had almost no audience in the United States. A port to the XBox would have been more feasible for the West, but the XBox wasn't at all popular in Japan, so there was no chance of that.
Additions to Nocturne in the Moonlight:
Despite the flaws, though, there were somethings that were added to the game that weren't a complete loss. For starters, two new areas were added to Nocturne in the Moonlight: the "Underground Gardens" and the "Cursed Prison". Although some have commented that the new areas weren't all that necessary, or that they added much to the game (oddly, the game itself computed percentage based on the PSX game, so these new areas threw the percentages off), they were welcome additions all the same. The Gardens attached to a secret, glitched areas at the "Entrance" of the original game, and was presumably intended to be included in that game. Meanwhile, the Prison was a set of hallways that connected the "Marble Gallery" and the "Caverns". It's a shortcut, basically, but considering the long load-times of the port, a welcome shortcut all the same.
The biggest feature, though, was the inclusion of Maria Renard as a playable character. Maria played a big part in original game, helping Alucard in his quest. Here, she's included as a playable character -- although, interestingly she plays different from her role in Rondo of Blood, more a martial arts expert than the bird-flinging, animal loving warrioress (her version included in DXC Symphony plays more like her original version). As a bonus, Alucard will also battle Maria once, where she tests him before giving him a key quest item. Both unlockable characters, Maria and Richter Belmont (included again, as in the original version), are playable from the outset, too, and don't need to be unlocked. Richter also has a secret second outfit more inline with his "villain" art from the promotional materials for the game.
Thankfully, although the Saturn port never came to the States, Maria was eventually included in DXC Symphony, meaning that the most essential feature of Nocturne did at least get an international release.