Again, Stage Hazard in SSB. You can't really compare it to other games because SSB has major differences for Bosses(they have quite a lot of attacks like Player Characters do) while Stage Hazards, Assist Trophies, Poke Ball Pokemon, and probably Special Flag summons were always reduced to one or two moves. It's apples and oranges. Every boss in the history of SSB literally has had a well over 2 attacks period. They've also been exclusive to Single Player Mode. Lastly, most of them have HP instead of percentage, but they also didn't move like a normal character. Giga Bowser is the sole exception, but he was also a clone of a regular player. Keep in mind I'm not counting the Metal Bros., Giant DK, or the various Fighting Teams. The first three are Mid-bosses(and clones of a regular character), and so are the Fighting Teams. Remember, no main Boss has been an actual clone before, except Giga Bowser, respectively. But he was also defeated like a regular character, so they are best called Mid-Bosses or NPC's at this point. That, and the only Boss Battles officially has characters that are featured on a stage(and can sometimes teleport/disappear temporarily) and all have HP. Brawl more or less defined what bosses are like.
The only thing I do see Yellow Devil having akin to a boss is the whole HP thing(although you could argue the Pokemon in Saffron City have it too, which is "1 HP"). The attacks don't show anything really special or unique, and his movements aren't very free like a boss. Bosses are scripted, but they don't have a set spot where they'll always appear every time, but they can still move around the stage in many different ways. Yellow Devil's movements don't match up with a boss' either. He just stands in place and shoots, along with body shifting to an area. I do not remember if you have to beat him to get him to temporarily go away(I also have a feel that people will use FD Mode for Wily's Castle more often due to him heavily affecting the match apparently).