In Season 2(I think) Gerald pretty much implied that Werewolves come from King Lycaon the king who fed Zeus his son Nyctimus to see if Zeus who was said to be all-knowing could recognize human flesh before he consumed it and when Zeus found out he cursed him turning him into the first werewolf.

In this story the history will change a little with all werewolves being descendants of different Wolf Gods from different mythologies and the wolf depends on were your family hails from.

All other creatures come from the bites of werewolves and the creatures you become is largely dependent on your personality.

Example Bloodlines: If your from a family of werewolves and your ancestry hail from the Denmark, Norway, and Iceland area nine times out of ten your a descendent of Fenrir, Skoll, or Hait.

Example Creature you become: In season 2 when Jackson was bitten he became a Kanima who is a creature who doesn't know who it is and requires a master to give it orders. Jackson took this from because he knew that he was adopted and he was also hiding the fact that he was gay and projected a fake persona so everyone believed he was something he wasn't.

Any werecreature can evolve and what happens during the evolution period is different but if you fight it for long enough you can stop and stunt you evolution.

Example: during season 4 it was implied that Scott was evolving but it was also implied that to complete the evolution he would have to take a life as every time he changed it was making him become more aggressive and when he finally died and was in the dream world his fangs changed and his fangs in the real world mirrored the change but after that he stopped going through the aggressive out burst during fights. If you look at the season as a hole Scott's evolution was him becoming an actual Alpha not just having the colors of his eyes becoming red but being an Alpha who was ready to lead a pack into war if he had to