I've always wanted to include Redcaps in my campaigns, have only used them once back in 3.5. I'm not sure Volo's Guide gives them a full enough treatment (there's only one type, fairly tough), but it's an idea... (e.g. minions of Baba Lasagna). Especially if your players mistake them for goblins...
And I'll second Yeth Hounds - again, something I've earmarked to include in another campaign.
Both are Fey, but so are Hags, and both are a decently tough challenge if you have a few of them.