1. I myself (possessive pronoun + self)
2. You yourself (possessive pronoun + self)
3. He himself (not possessive pronoun + self)
4. She herself (possessive pronoun + self)
5. It itself (possessive pronoun + self)
6. We ourselves (possessive pronoun + selves)
7. You yourselves (possessive pronoun + selves)
8. They themselves (not possesive pronoun + selves)
Whence this irregularity?
What an interesting question!
Although I don't think that's so simple…
Myself used to be meself actually.
Most forms had the dative (as in himself), there was also a difference between her (dative) and her (genitive), and the first one was gradually changed into the second one. The same thing happend with meself > myself.
This might not be 100% accurate so I'd suggest checking it in a trustworthy etimological dictionary.