You can change stats of your creatures as you wish, if you are the GM.
The examples that are shown in bestiary are just the basic member of a race.
There could be so many reasons for a creature to gain intelligence in a fantasy world.
But well if for some reason, you really want to do it with mechanics:
Add the savant template...then just follow the instructions to create npcs.
I was looking for a RAW way of doing it for piece of mind (plus I have a player who's a little bit of a RAW-Nazi, but to be fair he has calmed down on that quite a bit.)
That being said, I'm not one for third party material, but Green Ronin Publishing is a fairly trusted publisher, one of my favorites along with Goodman Games, and that is a very interesting and frankly, near perfect addition. So I may well make an exception. Thank you very much for letting me know about it.
The Advanced creature simple template from the Bestiary can be used to snag a +4 to Intelligence, and then the rest needed can be gained by milking the NPC stat array, and / or using PC point-buy.
That all said, Sorcerer seems like a more intuitive fit for a beastie spellcaster, and Charisma is easier to get up to good spellcasting levels.
I had forgotten about the Advanced template. Thank you. As to the caster type, it's because of a story element that I want to go Wizard with it. Thanks though.