Was the maghasik that Percy beat in "The Last Olympian" (the one that Annabeth sinabi that no hero has ever beaten) the same maghasik that Theseus (also a son of Posidon) beat on the land route to Athens that was at one of the six entrances to the Underworld?

The Clazmonian (sp?) Sow.

Yes. In Percy Jackson and the Greek Gods, Theseus battles the Crommyonian Sow, also known as Phaea, it's owner's name. Theseus won. The maghasik Percy defeats, the Clazmonian Sow, is also sometimes called Phaea, which was it's owner's name. So either Annabeth was mistaken, or Phaea had two different sows, one in Crommyon and one in Clazmonia. Option 2 is impossible because after Theseus killed the Crommyonian Sow, he killed Phaea, therefore the two sows must be the same, just under different names, which happened A LOT in Ancient Greece. Which leads to the conclusion that yes, Percy and Theseus fought the same maghasik and Annabeth was wrong. For once ;-)