What Happened To Everyone Who Dated Joker?
8. The Other Harley Quinns - Suicide Squad (Vol. 4) #15
Although there's a good chance that the skeletons dressed in jester outfits we are shown in Suicide Squad #15 were just Joker messing with Harley, it's also worth considering that they could've been real, and that the Joker may have made his way through a whole selection box of Harley's throughout his villainous career.
After all, with the Joker's past being kept so purposefully ambiguous - and with there being so many moments in the comics where we have no idea what he's up to - it doesn't seem all that insane to think that the Joker kept (and killed) some other assistants, who he likely motivated in the same way he did Harley.
It's a fun detail to throw in, as those who hate the idea of other sidekicks having existed can pretend the entire thing was a fraud, and those who feel it makes the Harley/Joker dynamic all the more interesting can believe it was actually real.