Giving monsters abilities that are typically restricted to character classes is something of a sticky wicket in the d20 System. Not because monsters can’t have such powers, but because giving it to them can be a cumbersome process under the framework of the game rules. What follows is a brief outline of ways to add class abilities to monsters, and the virtues and drawbacks of each method.
Hopefully this helps you give your monsters some class!
Adding Class Levels
This method has the monsters gain class abilities the same way that the PCs do: by taking levels in the relevant class(es).
The Good: This is the most straightforward method, with very little ambiguity since class levels don’t really interact with any powers the monster might naturally have. Even better, it works with every character class out there, no matter how unusual or outré, and there’s no problem if you want to use alternate class abilities or PF1-style archetypes.
The Bad: If you just want your monster to have one or two particular abilities, this method can be more trouble than it’s worth. If you want your rakshasa to have evasion and +4d6 sneak attack, you’re going to have to give them seven levels of rogue. This involves spending a bunch of skill points, giving them a couple of feats, recalculating their saving throw bonuses and BAB, along with a bunch of powers you didn’t care about them having. All of which is to say, this method can be a hassle.
Don’t forget that some monsters’ base stats are enough for them to qualify for certain prestige classes. Your average rakshasa, for instance, already meets all of the requirements to take levels in eldritch knight.
Variant Multiclassing
While it’s atypical to think of a monster’s natural Hit Dice as being like class levels, there’s plenty of history to the idea, from WotC’s Savage Species book for D&D 3.0 to Dreamscarred Press’s Monster Class line of supplements for PF1. And if natural Hit Dice are class levels, is there any reason why a monster can’t use variant multiclassing?
The Good: This method is one of the easiest to implement. You just eliminate the requisite number of feats (according to the chart on the page linked to above) and apply the matching class features from the class you’ve chosen.
The Bad: Even leaving aside that this relies on the PF1 feat progression (which is a relatively small hurdle to work around), the fact of the matter is that the rigid nature of what class features you get might not be precisely what you’re looking for. More than that, only a small number of PF1 classes even have their variant multiclassing rules outlined, and almost no other classes do, which can leave you stuck with having to write your own if the class you want isn’t on here.
It’s worth mentioning that Far Distant Future Publishing’s Variant Multiclassing Revisited solves a lot of the aforementioned problems. Not only does it rework the variant multiclassing system into a series of feats (albeit keeping the restriction that you can only have one variant multiclass), but it also has options for a large number of classes.
Simple Class Templates
Another PF1 option, this does what it says on the proverbial tin, turning classes into templates.
The Good: This feature combines the simplicity of variant multiclassing with the additive nature of taking standard class levels; the creature gains everything in the template (up to its Hit Dice) without giving up any feats or needing to mess with recalculating things like skill points and saving throws. It also helps that the CR adjustment is built right into the template.
The Bad: Since all of these templates involve an ability score boost (to the ability score primarily used by the class in question) some of the monster’s statistics will need at least minor recalculating. Moreover, the simplified nature of these templates means that a number of the powers in the original class are either absent or heavily watered down. Likewise, only a few classes have these templates made for them in the first place.
As with variant multiclassing, the third-party community has stepped worked to fill at least some of the gap where other classes are concerned. Rogue Genius Games, for instance, has published not just one but two supplements focused on simple class templates, while Fat Goblin Games has done the same for several prestige classes.
Gestalt Integration
Operating on the aforementioned idea that a monster’s natural Hit Dice are a class in-and-of themselves, there’s no reason why you can’t simply make a gestalt character out of them, integrating a class directly into their stat block.
The Good: Presuming that the creature has equal-or-better Hit Dice size, BAB progression, and skill points per level than the class you’re integrating them with, then this is one of the easiest options. Just add in the class abilities, and you’re done! And if the weapon and armor proficiencies they gain give you some new options, so much the better. Plus, you can customize the class if it has selectable suites of abilities (e.g. rogue talents) or PF1-style archetypes.
The Bad: If your monster doesn’t have better Hit Dice, BAB, or skill points per level than the class you’re making them a gestalt with, then this can involve some considerable recalculating of their stat block. Also, you’ll need to recalculate the monster’s CR on your own. If you’re using PF1, you’ll need to figure out if the monster gains favored class bonuses for its gestalt class.
For a slight twist on the gestalt option, check out the “amalgam characters” idea on this post from Owen K. C. Stephens’ blog. An amalgam character is essentially the same thing as a gestalt character, except “All of the abilities of both classes are considered to be native to the amalgam class. This can be important for rule interactions. For example, an amalgam magus/wizard treats all their wizard spells as being magus spells when determining if they can cast spells without suffering from arcane spell failure.”
Depowered Monsters
This idea turns monsters into monstrous equivalents of humans, elves, dwarves, etc. With no natural Hit Dice and special abilities that are reduced in power (if not removed altogether), they rely entirely on class levels for advancement. From an in-game standpoint, such monsters could be weakened versions of the standard monsters, being sickly mutants, runts with stunted growth potential, racial offshoots, or other variants.
The Good: This essentially sidesteps the issue of monsters with classes altogether, rewriting a given monster as a PC playable race. In that regard, they’re no more difficult to deal with than any other character, at least on a mechanical level.
The Bad: This option runs the risk of watering down what makes a monster notable in the first place, since its inherent powers are necessarily reduced to the point where they won’t break the game at 1st level. More than that, you’ll need to either find a third-party product with “PC monster” rules for the particular monster you’re looking for or write them yourself.
If you do decide to make a depowered monster, Pathfinder’s race builder rules might prove helpful.
Intelligence Check 