A drop is some critter that’s genetically predisposed to being very affected by a certain type of magic, and if they spend enough time around that affinity’s magic (either naturally in the atmosphere or another way) they’ll start to change. This doesn’t have to happen early in life, it can happen late in life (if, say, the drop in question simply had not been around a strong source of the magic that affects them in particular).
This is rather rare. A lot of drops don’t even know they fall into a category of creatures, they just undergo a change over a period of time and end up becoming something different. They’ll sometimes still call themselves by their old species name because that’s all they know to do (that is, if they can still remember it. Sometimes the changes are so radical that memories get jarred and personalities get changed).
This whole “not sure what species this is” thing applies to others looking in, too; drops can be mistaken for demons and imps because they look pretty strange more often than not!
Drops interact with magic of their affinity differently than most magic-wielders of that affinity. Differently how? It depends on their personality, but it’s usually tied to the ability to do extreme things with it; they exercise extreme control over that element. This also means they’re extremely susceptible to whatever weaknesses they have!
Also, they’re less “stable” as lifeforms than most critters, which lets them do interesting things with their bodies…