The hydrogen ion is the most abundant in an acid for it is a bare proton that associates with any water in the molecule, solution or reaction so the H+ ions produced by an acid exist as H3O+ ions.
Moreover, in the Arrhenius theory, an acid is a substance that dissociates to produce H+ ions in solution and a base as a substance that dissociates to produce OH- ions in solution. Meanwhile, in the Bronsted-Lowery theory, an acid is considered as a proton donor.