I belive you are referring to
The Mystery Goomba, from Super Mario 64, which if I recall correctly spawns below the death plane.
Part of a gang of three
More elusive than my brothers
Goomba's tend to spawn in groups of 3, but this one appears to be only 2 at first.
Get close to us and you will see
You can catch a glimpse of me
The Mystery Goomba is visible for but a frame after becoming active.
I'm an infamous enemy of lower rank
When you're near us, I'm visible for one frame
Goombas are low-rank enemies I guess, and this guy dies almost immediately.
When you leave my brothers, I don't exist
I'm untouchable, unless you play god
I think this guy is only reachable through some sort of edit mode? I don't really remember all the details out of the video I saw.
I am undefeatable by mortals, no matter what you do
You will never get my precious gold!
Mario can't reach the Mystery Goomba to kill him, and the coins he'd spawn are death are some of the last theoretical coins that haven't been collected.
You will never catch me alive!
For I am at death's door
Lasts only a frame, because he's below the Death Plane