In the Pokemon anime series, Ash Ketchum's original travel companions were Brock and Misty. Brock is the oldest of the trio, and his cooking is something that both his human and Pokemon friends appreciate. As he travels with Ash, he catches a number of different Pokemon, though he doesn't make much of an effort to expand his team.
Due to his past experience as a Rock-type Gym Leader, there are a few Rock-type Pokemon on Brock's team. However, he loves Pokemon of all types, and he catches a number of them while pursuing his goals of being a Breeder and Doctor. While some of his Pokemon go through their entire evolutionary line, others remain at their first stage.
Geodude
In Pokemon Red, Blue, and Green, Brock is the Gym Leader of Pewter City. He specializes in Rock-type Pokemon, which means that players who pick the Water-type or Grass-type starter are at an advantage. In the Pokemon anime, Brock is also introduced as a Gym Leader, so it is no surprise that Geodude is one of his signature Pokemon. It is on his team when he battles against Ash in Pewter City. Geodude is capable of evolving into Graveler and eventually Golem, though Brock is never shown evolving this partner.
Onix and Steelix
Another Pokemon that is strongly associated with Brock is Onix. During Ash's first battle against Brock in the original season of the anime series, Brock's Onix gives Pikachu a tough time, though Ash eventually earns his Boulder Badge by over-charging Pikachu's Electric-type attacks. In Pokemon the Series: Ruby and Sapphire, it is revealed that Brock's Onix has evolved into a mighty Steelix, which is a dual Steel/Ground-type creature. It makes an appearance a few times in later seasons, including in the Sun and Moon anime where Brock can turn his partner into a Mega Steelix.
Zubat, Golbat, and Crobat
Zubat is the first Pokemon that Brock catches during an episode of the anime series, although it does not happen on-screen. Fans familiar with the video games likely have a love/hate relationship with Zubat, as while the Pokemon is good, it can make exploring caves a pain in older titles. In Johto, Brock's Zubat evolves into a Golbat and eventually into a Crobat, which is a Posion/Flying-type. During all stages of its evolution, the Pokemon proves useful to Brock and his friends, as it is good at scouting for the villainous Team Rocket.
Pineco and Forretress
Zubat evolving into Crobat isn't the only interesting thing that happens with Brock's Pokemon during their trip across Johto. It is also the region where he catches his Pineco. It proves to be an occasionally troublesome team member due to its tendency to self-destruct. The Bug-type Pokemon evolves into Forretress, which is a Bug/Steel-type, and in this form it seems to mature. It no longer explodes as often, which Brock is grateful for.
Croagunk
Crobat isn't the only Poison-type creature Brock ends up using. He also catches a Croagunk, which is a Poison/Fighting-type Pokemon with frog characteristics. After being abandoned by Team Rocket, Croagunk takes a liking to Brock, and the two become inseparable. It may not be Brock's preferred type, but they still make good partners - even when Croagunk repeatedly pulls Brock away from his confessions of love. Croagunk can evolve into Toxicroak, but Brock's buddy does not.
Bonsly and Sudowoodo
Brock catches Bonsly in Ruby and Sapphire, and while it may seem out of place at first, this was just one of many efforts at the time to advertise Generation 4 monsters. In Diamond and Pearl, the cute Bonsly evolves into a Sudowoodo, which is a Rock-type like its pre-evolution despite the line's gimmick being they disguise as trees. Brock isn't the only Gym Leader who is a fan of Sudowoodo. In the recently released Scarlet and Violet, Gym Leader Brassius has a Sudowoodo on his team that he Terastallizes into a Grass-type.
Happiny, Chansey, and Blissey
In Diamond and Pearl, Brock receives an Egg. Brock takes care of it, and it eventually hatches into a Happiny, which is a Normal-type baby Pokemon. This is the perfect creature for Brock, who has a very protective and caring nature when it comes to his Pokemon. Through Brock's care, Happiny eventually evolves into a Chansey, and when Brock makes a reappearance in Pokemon Journeys it is revealed that Chansey has evolved into Blissey.
Lotad, Lombre, and Ludicolo
Brock catches his Lotad in the Hoenn region after rescuing it from Team Rocket. It often teams up with Brock's Mudkip, and Brock's Lotad eventually evolves into a Lombre and then a Ludicolo, which is a Water/Grass dual-type. Although the Pokemon can be a bit dim-witted, Brock is very patient with it. In Ruby and Sapphire, it is revealed that he left the Ludicolo with his siblings after seeing how much they love it.
Mudkip and Marshtomp
Brock receives Mudkip from Old Man Swamp, and the two quickly bond. When Brock and his friends are confronted by Team Rocket, Mudkip usually pairs up with Lotad, and they combine their Water-type moves. Mudkip evolves into Marshtomp while Brock is having a friendly battle against Ash. The Water/Ground-type Pokemon is powerful, though Brock chooses to leave it at the Pewter City Gym when he travels to the Sinnoh region.
Comfey and Vulpix
The last two entries were both gifts. Brock receives Comfey from Nurse Joy in Alola, and Vulpix is a Fire-type fox who technically belongs to Suzie from the Kanto region. She gives him the Pokemon, as she has faith that he will good care of it. He eventually returns the Pokemon to her.
Overall, Brock catches or receives 11 Pokemon lines, which is much fewer than characters like Ash and Goh. This isn't a problem as his goal isn't to be a Pokemon Master or to complete his Pokedex. Instead, Brock wishes to be the best Pokemon Breeder and find the best ways to care for these creatures. The few Pokemon Brock does have, he treats with kindness, which makes him an important representative of the Pokemon franchise's goals and lessons.