It is possible to have the trophy for an inaccessible boss. To do so, the player must defeat the normal version of the boss (unlocking the Battle Ring version), then defeat the X boss without saving. After the battle results are saved, the player must reset the game and go back to the Battle Ring. The trophy for the boss is viewable, but the boss is not unlocked yet.
All giant X Bosses have the highest possible speed in the game.