Is there a special unit for that, like Zerg eggs? Or is it just detecting Archons?

It's just detecting Archons, I believe. I don't get why it would be detected with a zerg egg .

I ment did they have a seperate unit like Zerg eggs.

No, its just a pure archon.
What he ment was how the zerg have 3 or 5 stage evolutionary. Where the larva would morph into a mutalisk which would morph into a guardian.

It's just an Archon, just like the name of it would say while merging, that's why they play the end of the merging animation at the start of a map for preplaced Archons.

And when spawned, that's why they look best for non-bound explosions

You could have done this yourself, yunno. Tested it.