Hello friends, I've recently come across an issue in a campaign I'll soon be joining. I'm very interested in having my character be a devout follower of Zon-Kuthon, and as such eventually pick up Leadership and get preferably an Evangelist Kyton and possibly an Erinyes eventually as a cohort. I've looked into it a lot, but it doesn't seem there are actual elastic rules for it - short of speaking at length about each one specifically with my DM. Does anyone have prior experience with this, or even better has Paizo spoken more clearly about this anywhere?
http://paizo.com/PRD/monsters/monsterCohorts.html
http://paizo.com/PRD/monsters/monstersAsPCs.html
Those two pages are pretty much all you have to be honest, but with some work with your GM, it should be enough. If you treat the kyton as a PC using the rules from the second link, you should have a reasonable approximation of power. Obviously, your GM will need to look over things and make sure nothing is blatantly OP.
If you want clean and clear rules for monster as PC style cohorts? No, nothing that clean I'm afraid.
I personally have experience with it and my personal experience has been that, in general, it is best to weaken the abilities of monsters and treat them as PCs a bit more. Lich PC? Replace it with stunning fist feat except paralysis instead of stun. That lets it stay relevant even into high levels, but isn't blatantly underpowered at low levels.