How do you determine what level characters monsters are equivalent to in Pathfinder? I'm going to be playing a Pathfinder game in the future and I am playing as a monster PC. The GM gave me a few examples of appropriate choices like mummies and werewolves. (It will be an evil campaign). I need to know what I can pick as a monster race that is appropriate for a level 8 character.
Pathfinder provides no specific guidance on this.
In D&D 3.5, there was something called a level adjustment (LA); this was a gauge of how powerful a monster would be as a PC. It never worked very well; the power of the PC depended heavily on the class and build you chose, and LA had to account for the most powerful combination.
Additionally, many monster powers that are only somewhat useful in combat turn out to be quite powerful in the hands of PCs. This meant that the HD+LA of a monster was often higher than it's CR.
Pathfinder removed the whole idea of LA as unworkable; they dropped the pretense that there was an easy mechanical solution to balance a party that mixed players and monsters.
I think your best bet for guidance is just to look for cool things to play, and ask the DM if you can use it. But anything with a CR higher than 8 is probably too powerful.
Some monsters are actually templates (such as Vampire and Werewolf) that you add to a normal character created with normal rules. Make sure your GM allows actual monsters.
If it's a template, on the template itself you'll follow instructions on how to turn a normal character into, for example, a Vampire. Vampire template is CR+2. The appropriate CR for a party level 8 is CR8. So build a character level 6 and add the Vampire template.
If your GM actually allows monsters look at the monster list and check for a CR8 monster. Make sure your choice fits with that the GM has in mind (because if you play a Dinosaur in a Werewolf+Vampire game that might be weird). Efreeti could be fine but a Dire Tiger...not sure.