Playing the monster
Since the dawn of dnd there have been players who want to play the monsters. As those creatures became more in depth, interesting, powerful, and diverse that desire has only grown stronger. One of my favourite books in my entire collection is the classic "Savage species" which, while dated, is still a great source of inspiration for me. With the dawn of pathfinder there was a great deal of hope about being able to play as monsters and it was met with a rather perplexing pair of solutions.
First you can let everyone be monsters of that challenge rating. This solution is much closer to fair as it allows everyone the chance to get to be nice and monstrous and get some neat abilities, although it can be a little challenging to find many monsters of the same cr that are suitable for play it's hardly impossible if you have an expansive enough bestiary. This is doubly true if you bear in mind the advice for using 3.5 content and just lowering the cr by 1 or slapping on the advanced template to make it pathfinder equivalent. This is a good option if you don't mind everyone being strong, but equally so, and everyone wants to be a monster.
If just one player is looking to be a monster you let them play as that base creature and count it's cr as levels, and then every 3 levels beyond you let them gain 1 extra level between the second and third. repeat again until you have gained bonus levels equal to half the monsters CR. Woah, what? Really? Yeah thats what it says. So I'm gonig to take a look at a few examples of this in play. You can skip the italicized section if you don't need illustration of why this might not be balanced.
Example 1: "The Redeemed" Your campaign is kicking off at level 8, or just a player is rolling up a new character at this Point. They want to tell the tale of a fallen hero struggling for redemption. They decide to play an Erinyes. Perfect it's CR 8. She has 9 hd which gives her 9 base attack, and more skills than the fighter. Some DR, spell resistance, flight, some immunities and amazing ability scores along with constant true seeing and greater teleportation at will. Both of which are powerful abilities that the cleric and wizard don't have access to even once per day yet. Wow, that's pretty strong but let's say the gm approves it. The story progresses well and soon it's time to gain a level. Of course the Erinyes becomes a paladin, turning the good side and swearing faith to a goodly god. That's not too bad, base attack is still great, she can fight with the best of them, and once a day she can add her huge charisma to attacks and ac against evil. Not that her ac wasn't good enough. Fast-forward another level. Divine grace, lay on hands. self healing adding charisma to all saves. Getting intense. And then the Erinyes gets another level before anyone else. aura of courage, divine health, mercy. She's now got 12 HD, and 12 base attack. The fighter has 10. The campaign progresses all the way to 20th level. Our Erinyes paladin is now a 16th level paladin. She has 25 base attack, or perhaps 23 if you follow the old rules for epic levels. Her saves areprobably 17/7/16, or again 14/7/14 with epic rules plus adding charisma to them. Woah.
Example 2: "The solar master" Ancient Solar dragon Sorc 2. Sometimes a DM wants to run a nice long level 20 game, maybe going through many mythic trials at the end to provide progression. And one player isn't satisfied with a sorcerer who merely has traces of draconic blood, they want to play a glorious brilliant full dragon. Specifically an ancient solar dragon with two levels of sorcerer. Sure, why not? That's cr 20..I guess. Now the party wizard has some concerns. Mainly that the sorcerer has 27 HD, 37+ac, more than 300 hp, and a higher attack bonus than the fighter, better saves than the paladin, and a 20d10 breath weapon. Yes, that's all very true. But hey, the wizard has 9th level spells, and CL 20. The dragon only has 8th level spells and cr 17. Unless it takes practised spellcaster for a feat...which makes it's CL 21 but hey at least they didn't get free sorc levels every 3 levels.
Example 3: "The superhuman" So you're running a level 5 game. and your player tells you they're really excited to play a really cool super awesome unchained monk. Nice nice. Let's see what they've got. Monk 1....advanced human 4?as in the advanced template? Well I guess. I mean it has a cr....wait. advanced x4? so they have +16 to all ability scores? And +8 natural armor. But they add dex and wis to ac, so that adds +24 to their ac overall, ontop of base 10 is 34 ac....and +8 on all their saves. They get +8 attack and damage. A 5th level monk only has 5 base attack. But wait, hold on he's only got like 10 hp, well 10+con, which is at least 8. But yes, he's got low hp. But every level he gets more. And you have to hit him to deal damage. Or maybe not right? reflex saves for half damage , half damage is still great...till level 2 when he gets evasion. Then before everyone else his 7 he hits level 3, and then hits 4 when they hit 7. by level 10 his 4 advanced templates cost him 2 levels. When everyone else is 20, he's 18, with +16 to all his scores.
Ok, ok, enough math, enough examples. I'm sure you get the idea. Now of course a few things should be said. It's not always going to be that strong, many players aren't that great at optimizing, though many are even better. And yes the bestiary explicitly states to keep a close eye on using this rule, but from that starting point there will always be problems. And on top of that there's no explanation for how much of that monster's stat block one should use. For example, does the Erinyes get the elite array for ability scores? She's a player character after all. Does she get starting equipment like the players? she'll definitely start getting equal shares once they begin play either way. Can the solar dragon pick different feats and skills? There's a lot left unanswered. So without further ado I'd like to present my solution:
If you want to play a monster right from the bestiary with no changes it is treated as a character with class levels equal to it's cr + 1 for every 5 cr or part there of. A cr 4 monster is a level 5 character, a CR 8 monster is a level 10 character, cr 19 is a level 23 character and so on.
If you allow the player to fully customize their monster, including adding any template, do as follows: Subtract 10 or 11 from each of the monsters ability scores to get an even bonus, and then let the player use the same ability score generation as the other players. Whether that's rolling, point buy, elite array or some other method. and then add the bonuses back on. The player can reselect any feats they get from their HD, but not racial bonus feats, and they can re-allocate their skill points and get gear appropriate for a character of their level. and if youallow the player to choose their own feats and skills and give them level appropriate gear. In exchange for this customization further increase their effective level by 2 more. This means that a CR 5 troll would be playable and fully geared out fighting alongside a team of level 8 heroes. A cr 11 barbed devil with free reign over feats and a full set of gear is matched up with level 16 heroes (who can likewise teleport and can be boasting spell resistance. And the strongest monster that can be played at level 20 is cr 15, which still allows for many mighty dragons albeit with far less magic than their CR 20 counterparts. This system greatly reduces the liklihood of players having more HD than their team mates and helps bring their abilities into line with what other party members may acheive, and it also account for player who are customizing and optimizing their choices vs ones who just want to open up the book and play a monster.