How can I get the PC to transform from himself into, for example, a wolf? I have seen mods that use this, but I don't understand how they work. Can someone help?

Easiest Method is to turn the Wolf (or any creature) into a Mount (Requires Either Andrew Maneri's Mount Animation Wrapper or a 3D App to convert the Skeleton and Animations to the Bip02 classification), then you just use an X-Form node before the ActorParent Node and set its scale to 0 to shrink the entire Child Hierarchy to basically nothing.

Going the Mount route is easiest because a great deal of the Oblivion Creatures have already been converted into Mounts by others so they are already available, it works in both POV's and its fully supported by the game engine, its the only method that does not require OBSE but can be made more functional with its use.

Another method is to just spawn a creature at the player -> make the player invisible -> then force the creature to play animations based on what the play does <- does not work worth a damn in 1st person.

Another method is to use OBSE and alter the Skeleton Path then force the player to equip a creature costume <- this method does NOT WORK AT ALL in 1st person.