Nighttime-exclusive Minibosses

It's always been a long-standing Minecraft rule that once night falls, you stay indoors. No matter how much armor you've amassed or how well you know the lay of the land, exploration at night is kind of pointless - why risk the danger of running into mobs when you could simply wait for the daytime for a safer voyage?

I suggest an elegant solution that would entice well-equipped players to venture beyond their well-lit bases and mob farms - special minibosses that spawn at night in remote areas.

These minibosses would be very rare mobs (or groups of mobs) with two things in common: One, they are VERY tough to take down. Two, defeating them will grant rewards well worth the effort. They would typically spawn a decent distance away from light sources, so you wouldn't have to worry about one appearing by your base - these nighttime terrors will only be found by explorers bold (or foolish) enough to venture deep into the darkness. They could be anything from Giant Spiders to Skeleton Centaurs to entirely new mobs with unique properties and drops. Perhaps some minibosses would only spawn on certain moon phases! 

These types of mobs would add more unpredictability to Minecraft, as well as offering veteran players additional nightly challenges so that gameplay never gets stale. They would revolutionize the nighttime, making it even spookier and more treacherous while also providing a reason to risk a nighttime stroll if you've got the armor, weapons, skills, and thirst for adventure.