From Bulbapedia, the community-driven Pokémon encyclopedia.
Hoenn first partner Pokémon, otherwise known as Hoenn starter Pokémon, are Pokémon chosen by Trainers at the start of their journey in Hoenn. These are traditionally given out by Professor Birch and are to be used to battle the first wild Pokémon that the Trainer encounters.
In the core series Hoenn-based games, the player can choose a Fire-, Water-, or Grass-type first partner Pokémon at level 5.
Availability
Hoenn-based games
In Pokémon Ruby and Sapphire, Emerald, Omega Ruby and Alpha Sapphire, after Professor Birch is rescued from a PoochyenaRSORAS or ZigzagoonE, he allows the player to keep the Pokémon they chose to rescue him with.
The player's neighbor, Brendan or May, already has their first partner Pokémon, which is always the one with the type advantage. By the end of Ruby, Sapphire, and Emerald, it seems that the player's nominal rival, the son/daughter of Professor Birch, has stopped being a Trainer, and Wally is actually the true rival, facing the player before they can leave Victory Road and staying there to have rematches with the player later. In Omega Ruby and Alpha Sapphire, Brendan or May's role as an NPC is slightly expanded, acting as a post-credits battle in which they gain access to Mega Evolution for their first partner Pokémon.
Other games
In Pokémon HeartGold and SoulSilver, Steven Stone offers one of these Pokémon to the player after they have defeated Red.
In Pokémon Ultra Sun and Ultra Moon, the middle stages of the Hoenn first partners can be caught in the wild via Island Scan on Akala Island.
In Pokémon Sword and Shield, both the middle and final stages can be caught in Dynamax Adventures in the Crown Tundra.
In Pokémon Brilliant Diamond and Shining Pearl, the Hoenn first partners can be caught in the Grand Underground after obtaining the National Pokédex.
Base stat comparison
The Hoenn first partner Pokémon; owned by Brock, Ash, and May from left to right
In the anime
in the main series
During the Hoenn arc of the main series, Professor Birch gave away four of the three first partner Pokémon to new Trainers. Harrison and May both received a Torchic and Sawyer began his journey with a Treecko. Although Professor Birch isn't seen gifting a Mudkip to any Trainers, in A Mudkip Mission, Mudkip Breeder Old Man Swamp revealed that Professor Birch is also responsible for the breeding and raising of the young Mudkip for new Trainers.
In the manga
Ash & Pikachu
In the Ash & Pikachu manga, Professor Birch gives a Torchic to May.
Pokémon Adventures
In Pokémon Adventures, Professor Birch gifts a Torchic to Sapphire and a Mudkip to Ruby. Professor Birch was also revealed to be in possession of a Treecko, however the Pokéball containing the Pokémon was washed away in a flood. It was later found by Wally, and eventually came to be under the ownership of Emerald.
Pokémon Pocket Monsters
In Pokémon Pocket Monsters, Professor Birch is in possession of the three Hoenn first partner Pokémon to which he gifts to new Trainers. The only one he is seen giving out though is a Torchic which he gives to Sapphire.