Ash's Pokémon in the Hoenn region are Pikachu, Taillow/Swellow, Treecko/Grovyle/Sceptile, Corphish, Torkoal, his original Bulbasaur that he left back home and Snorunt/Glalie.
pikachu(obviously)
toarkoal
corphish
treecko(which evolve into grovyle then sceptile)
snorunt(which evovle into glalie)
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The Hoenn Region is available in the pokemon versions Ruby, Sapphire, and Emerald.
You can't get into the hoenn region in Pokemon diamond. It's a 1 region game and that region is SINNOH! BY THE WAY HAVE YOU HEARD OF POKEMON X AND Y
Hoenn Region
No. You CAN import Pokemon from there, though.
No so far the hoenn region is exclusive to Ruby, Sapphire and Emerald version.