The Wey-Yu Report says that it was most certainly an Engineer.

I believe the RPG says there are in universe theories from xenobiologiest saying it might be an older race whom the engineers took from.

I 100% believe Ridley wanted it to be that the space jockey was an engineer however I find it more interesting that there's still a bit of mystery.

It's also just possible that it is an engineer and either they vary in size more than we have seen so far, or it's an ancestor to the engineers whom are bigger.

Either way it's fun to theories.

Since that example was destroyed, it's impossible to know for sure.

Just as an extra tid bit there are some other aliens that have appeared.

The Reapers, who appeared in Aliens: Reapers.

There's an unnamed Alien pilot in Alien Earth Angel.

And Ancient Martians from Aliens/Vampirella.

Couldn't tell you how canon any of those are but still.

None of those are canon.

The Fulfremmen were also created by the Engineers. Engineers and humans can also become Fulfremmen if they're infected by the 26 Draconis strain of the pathogen.

Reticulans (Greys), but that's just in my headcanon.

Is the Alien Beetle in the beginning of the special edition of Alien: Resurrection canon btw?

And is the “Alien” in its name referring that it is smth extraterrestrial or Xenomorph

Everything shown in the films is canon. The only thing that's non-canon about the bug is Distephano squashing it in the novel, since it's an unnamed crewmember who does that in the on-screen version.