With all due respect, those are largely pretty small differences. It’s easy to say that the size of the plume or legs or the thickness of the neck are, all things considered, kind of minor details. Those things easily vary from depiction to depiction.
This is a 3D model, remember. The beak wouldn’t show because it would be facing forward. The same applies to the explosive tail; it’s true that the tail is huge, but because the sprite is in 3D, most of those feathers are obscured by the rest of the silhouette; those feathers that can be seen definitely resemble Ho-Oh’s. There’s also the fact that Ho-Oh was shown during Smash to consider. I doubt they’d showcase Ho-Oh randomly like that.
As for the bumps on the wings, that I can admit I have no explanation for. Maybe it’s some aspect of the targeting system? I’m not sure. Ho-Oh definitely does not have wing-bumps. Still, I think there’s a lot more evidence for the bird being Ho-Oh than you think. Again, though, this is really just idle speculation.

I’m not trying to be rude or anything; but here I am looking at a few pictures of Ho-Oh and I still don’t see the resemblance at all.
The plume on the Mystery Bird is much to large to be Ho-Oh’s, if it even is a plume on its head and not its head itself.
Ho-Oh’s legs are skinnier than that of the Mystery Bird.
Ho-Oh doesn’t have bumps on it’s wings.
On the Mystery Bird, if it really is Ho-Oh: Where is Ho-Oh’s massive beak? It would be sticking out somewhere.
The neck of the mystery bird Pokemon is too thick to be Ho-Oh’s. The neck and head shape all together are too thick to be Ho-Oh.
And lastly, Ho-Oh’s tail does not stick straight out behind him…. I don’t even understand that one. Have you seen Ho-Oh? He has a large, explosion of feathers behind him for a tail. It is a huge tail, that would be the most distinguishing feature in a silhouette.
Ho-Oh is just not a possibility.

Side note: I noticed that these look like Crustle (tucked in their shells) blocking your path… Interesting…

I’m surprised you think so. The giant plume on the mystery bird’s head strongly resembles the one on Ho-Oh’s. It has the same claws. Its wings are similarly segmented. Ho-Oh’s tail sticks straight out behind it; the mystery bird’s tail seems to do something similar. And I think it’s not a coincidence they’d show Ho-Oh all of a sudden in Smash. It’s just a theory, but really, I think the resemblance is pretty strong.

But isn’t it bad that the Magmar you trained carefully goes to the arms of a stranger and what turns back is that stranger’s magmar and evolution, you don’t know how it’s trained, who is its trainer.

This literally made me laugh out loud, and I can’t tell if you’re trying to be sarcastic or not.

Ho-Oh is like the least likely bird Pokemon it could be. No resemblance.

I dont know if i should choose magby or growlith?!