So, these guys. One of the most hated Mega Man enemies, and for good reason.
They appear from holes just as the player is trying to jump them, and have caused uncountable pits deaths by either surprising players into falling or, due to a quirk of the game's physics, causing Mega Man to be pulled into the hole when hit if the player tries to turn around.
They're also a rare enemy, allowing me to cover every instance of their use right here.
Up'n'Down was introduced in Dust Man's stage, and our first encounter is reasonably fair. It appears quickly enough that a player unaware of them will likely run into it and be knocked back before the pit becomes a danger.
Each hole in this area contains one, so there's no guessing involved once the player has seen the first. The last two also usually move high enough to be jumped under, and players have plenty of time to practice shooting them as they appear.
I actually have no idea what determines the height they move to, and it seems random from my experiment here. This segment later in the game is more dangerous, but the tall pillars and lack of other enemies is a good clue that they'll appear, and our height makes it possible to react as they fly upward. The pillars also give us just enough room to jump each one at a run, making this a fun segment for experienced players, while novices have time to react and the tools to make this easier if necessary.
The most memorable segment involving these comes in the first Dr. Cossack stage. The slippery floor makes this extra dangerous, and while we can barely jump most of them, we need to stop and shoot the one in the third image.
That said, this area is still handled well. We have a high ledge at the first one, making it easy to dodge, and each Skeleton Joe that could reach a player waiting at an edge can also be shot from there, so nothing can force the player to hurry. The lack of a ceiling or flying enemies even makes it easy to bypass the whole thing with Rush Jet.
Mega Man 5's version appears in only two places, and it's practically decorative in Star Man's stage due to the high platforms and low gravity. In Napalm Man's stage, we can see that Mizziles always move to the same height and are a bit faster. This means we can jump under it without having to watch where it moves to, and a player getting hit by one is less likely to be knocked into the pit, since they have less time to turn around. Mizziles only jump up again after they've moved back down, so we don't have to worry about a second one popping up as soon as we think it's safe.
This version was fairer to the player, and it's unfortunate that we didn't see a little more of it. This segment used it as well as can be expected though, forcing us to wait for them in the cramped area at first glance, but allowing players to bypass them quickly with slides. The empty pit at the end is also a fun fake-out to pull right before the boss door.
In Mega Man 6, Gabgyo (the version that jumps from pits anyway) is nearly exclusive to Plant Man's stage, with a small appearance in Yamato Man's. The fact that it jumps from water gave us a clue to watch for them, and Yamato Man's stage-- where water was an unexpected thing to see-- was the only place that tested the player on this.
On the other hand, Gabgyo was not the only enemy on this screen, and players may not have seen them in Plant Man's stage yet, so this room may have worked better in the Dr. Wily stages. I've covered this section of Plant Man's stage thoroughly, so I'll only add that its first appearance is the most dangerous introduction this enemy type has had in the NES games.
Despite that, the stage design and other enemies support it so well that this remains my favorite use of these guys.
While the first game also had a couple enemies that appeared from pits, and Mega Man 6 included enemies that rise from platforms instead, the only other instance of an enemy truly similar to Up'n'Down was the Big Fish in Mega Man 2. This was mostly for show, given Mega Man's jump height in water, but was very effective at putting the player on edge.
Overall, while these are a nasty enemy for new players to deal with by design, there were no lazy uses of them, no random placements in the middle of a group of pits. Most were confined to their own segments or, in the cases of Star Man and the Big Fish, kept out of the players' expected jump arc. The one most likely to kill a player without warning was in Yamato Man's stage, and then only because a player may not have been tipped off to watch the water before trying it.