Arceus is the alpha Pokémon.what's the beta Pokémon because if I'm correct the beta Pokémon would be Arceus second,not as powerful but is still important.
It's not necessary to have a beta Pokemon just because there is an alpha. Dialga looks similar to Arceus so consider Dialga as the beta Pokemon. Dialga may not seem important but I consider all legandaries important.
I don't really think there is a "beta Pokemon" that GaneFreak came up with. The only thing that pops into my mind is Dialga or Palkia. Other than that, there is no official beta Pokemon.
Alpha in this case refers to the Alpha male or simply put the King (or queen), meaning that Arceus is the King of Pokemon. It doesn't mean Alpha like the one in the Greek Alphabet.