Twice now, one of my players used "Remove Curse" on beings which I don't know if it would effect. Curses which change you or other creatures seem ill defined as to whether this spell would work. Would using the spell on the following return a being to their normal state:
1. A Were-creature
2. A Drider
3. A Nightmare that was once a Pegasus
4. A Yuan-ti
It seems a bit over powered powered that such a low level spell could fix these, but I have watched at least one WoC run game where it fixed Lycanthropy.
Among the examples you mentioned, only Lycanthropy is a curse, and the description in the MM specifically says that a Remove Curse cures Lycanthropy (except for natural-born lycanthropes, which requires a wish spell).
The others are not curses, unless, of course, the DM says otherwise.
As Filcat says. In general, (as with many things with 5th edition), if something is a curse, it mentions so.
Examples include the Lycanthropy, cursed magic items (clearly marked as such in the Dungeon Master's Guide), effects such as those by Bestow Curse or Hex, or any effect the DM deems as such (the Pegasus-to-Nightmare scenario above seems like it could fall to this category).
So the differnce is the method of transferance? Nightmare description says that a ritual can turn a Pegasus in a Nightmare by ripping off their wings. Similar altered states, such as the blood ritual to become a Yuan-Ti, what would these be classified as if not a curse in terms of game mechanics?
Much like how things which are meant to be mechanically treated as "magical effects" have their mechanical wording include explicit mention of being magic, things which are curses - and thus removable by the remove curse spell - use the word "cursed" in their mechanics.
So a thing like mummy rot from a mummy's rotting fist or lycanthropy from a werewolf's bite is a curse, but things like driders, yuan-ti and nightmares that don't have the word "curse" anywhere in their stat-blocks are unaffected by remove curse (because, like not all rectangles being squares, not all instances of magical transformation into something monstrous are curses, even if someone might say "that creature lives a cursed existence" or something like that about it).
...what would these be classified as if not a curse in terms of game mechanics?
They would not be classified in terms of game mechanics. If any classification is needed at all, it would be "plot magic" - which is to say stuff that is how it is for the sake of the story created by it being so, with no intention of specific mechanical interactions being assumed by the game rules to be possible.
Also keep in mind that not all curses are magical. Neither lycanthropy nor mummy rot mention the word magic anywhere in their description, so it's not going to go away in an Antimagic Field or be picked up by Detect Magic. On the other hand, a curse from Hex or Bestow Curse is obviously magical, since spells are magical.
If there's a way to "fix" or revert a creature back to its original form, it'll be stated in its description; otherwise, you can assume a Wish will do the trick unless the transformation was caused by a god. If your players really want to fix a Nightmare, you can send them on an adventure to find a long-forgotten ritual that can do it, or try to enlist the help of a powerful entity like a fey lord. The best quests are the ones that your players personally have a stake in.
Twice now, one of my players used "Remove Curse" on beings which I don't know if it would effect. Curses which change you or other creatures seem ill defined as to whether this spell would work. Would using the spell on the following return a being to their normal state:
1. A Were-creature
2. A Drider
3. A Nightmare that was once a Pegasus
4. A Yuan-ti
It seems a bit over powered powered that such a low level spell could fix these, but I have watched at least one WoC run game where it fixed Lycanthropy.
Among the examples you mentioned, only Lycanthropy is a curse, and the description in the MM specifically says that a Remove Curse cures Lycanthropy (except for natural-born lycanthropes, which requires a wish spell).
The others are not curses, unless, of course, the DM says otherwise.
As Filcat says. In general, (as with many things with 5th edition), if something is a curse, it mentions so.
Examples include the Lycanthropy, cursed magic items (clearly marked as such in the Dungeon Master's Guide), effects such as those by Bestow Curse or Hex, or any effect the DM deems as such (the Pegasus-to-Nightmare scenario above seems like it could fall to this category).
So the differnce is the method of transferance? Nightmare description says that a ritual can turn a Pegasus in a Nightmare by ripping off their wings. Similar altered states, such as the blood ritual to become a Yuan-Ti, what would these be classified as if not a curse in terms of game mechanics?
Much like how things which are meant to be mechanically treated as "magical effects" have their mechanical wording include explicit mention of being magic, things which are curses - and thus removable by the remove curse spell - use the word "cursed" in their mechanics.
So a thing like mummy rot from a mummy's rotting fist or lycanthropy from a werewolf's bite is a curse, but things like driders, yuan-ti and nightmares that don't have the word "curse" anywhere in their stat-blocks are unaffected by remove curse (because, like not all rectangles being squares, not all instances of magical transformation into something monstrous are curses, even if someone might say "that creature lives a cursed existence" or something like that about it).
Also keep in mind that not all curses are magical. Neither lycanthropy nor mummy rot mention the word magic anywhere in their description, so it's not going to go away in an Antimagic Field or be picked up by Detect Magic. On the other hand, a curse from Hex or Bestow Curse is obviously magical, since spells are magical.
If there's a way to "fix" or revert a creature back to its original form, it'll be stated in its description; otherwise, you can assume a Wish will do the trick unless the transformation was caused by a god. If your players really want to fix a Nightmare, you can send them on an adventure to find a long-forgotten ritual that can do it, or try to enlist the help of a powerful entity like a fey lord. The best quests are the ones that your players personally have a stake in.
Thanks for the side quest idea, and to all who have joined this discussion.