The three legendary birds (Articuno, Zapdos, and Moltres) are puns on elements, as well as numbers in the Spanish language. Articuno is Arctic (ice) and uno (one), Zapdos is Zap (electricity) and dos (two), and Moltres is Molten (fire) and tres (three). This is also the order that they appear in the National Pokédex.
Contributed by gamemaster1991