What is the origin of prions?
The computational study by Ehsani et al., 2011, argues that an ancestor metal-ion transporter gene ZIP served as the initial template for the prion founder gene, where the same author produced a 2012 paper that said this was probably a product of a spliced LIV-1 ZIP transcript that became reverse transcribed and inserted into the host genome. Thus, evidence in part points to a retrogene origin for prions (mediated by a retrovirus, especially since the gene is integrated into the host w/ no introns). They go on to state that two genomic rearrangements spanning hundreds of millions of years serve as the basis for the prion founder gene: A cysteine flanked core element inserted into the ZIP ectodomain, and then the insertion of a spliced, C-truncated ZIP transcript.

I don't think the evolutionary origin of prions are very well understood, yet, but I will try to find more recent data on the subject.