Board Thread:The Panopticon/@comment-6032121-20190914173756/@comment-37902280-20200828151904

Although there's no real hard evidence to support this, my personal headcanon is that the Dalek Prime's varying accounts occurred due to the effects of the Last Great Time War: it started as the Dalek Prime from the novelisations / War / Legacy of the Daleks, then had its past rewritten so that it became a Dalek rank rather than an individual (like in The Four Doctors), then got rewritten again to become the Prime Dalek in Only the Monstrous.

On a side note, Nicholas Briggs voiced the Dalek Prime in The Four Doctors, so I'd imagine he's familiar with the term "Dalek Prime" as something more than just a word reversal of "Prime Dalek". Then again, there's a 5-year gap between The Four Doctor and Only the Monstrous, so maybe he forgot.