Artanis is able to merge his high templar into archons in Co-op Missions. Once Artanis reaches Level 7, his archons are upgraded into high archons, which retain their high templar abilities and gain an energy bar to use them. Researching an ability for Artanis' high templar automatically researches it for archons as well.
The following section contains information from a previous version of StarCraft II which is no longer valid.
Archons were in the StarCraft II game engine as early as 2006. They were much bigger than their final forms in this build, and were formed by merging three templars together.
The twilight archon existed in early builds. Later on the unit was replaced by the standard archon.
During development, the archon underwent a number of visual revisions. Its armor design was one such example, as it had to stand out from the energy effects. The legs and high collar were also removed, in order to give the archon a more fantastical look when compared to normal protoss.