The Pokémon anime has been running for a long time, and both human characters and Pokémon have come and gone. While all of Ash's traveling companions have Pokémon that they constantly use, many of them have Pokémon that are rarely seen, being used only once or not at all. Still, no Pokémon is as rare as Brock's Tauros, as he is never seen using it.
Episode 35 of the Pokémon anime, "The Legend of Miniryu", sees Ash, Brock and Misty arrive at the Safari Zone, a wild Pokémon preserve. In this enclosure, trainers are given 30 Safari Balls with which to catch Pokémon. Ash and his pals explore the zone and spot numerous Pokémon. Whenever Ash attempts to catch a Pokémon though, something happens and he accidentally catches a Tauros instead. By the end of the episode, Ash has used 29 of his 30 Safari Balls on Tauros. However, the last ball was used by Brock, who catches a Tauros of his own.
If this episode doesn't sound familiar to you, this is because it's one of many banned Pokémon episodes that were never dubbed or shown in America. The warden of the Safari Zone, Kaiser, carries a revolver, which he is seen brandishing at several points during the episode. Famously, in the first act, he aims the gun at Ash's head while explaining what happens to people who break the rules in the Safari Zone. Later in the episode, James is also seen holding this firearm to Kaiser's head to keep him still while Team Rocket interrogates him about the location of a Dratini. Obviously, U.S. networks heavily disapproved of this dangerous gun use and refused to air the episode, so it never got a translation and hasn't been seen outside of Japan. This means that, to American viewers, Ash randomly acquires a whole herd of Tauros with zero explanation.
But why is Brock's Tauros never seen again? At the end of the episode, Ash sends all of the Tauros he's caught to Professor Oak's lab, causing the poor professor to get hit with a massive stampede and suggesting that Brock's Tauros was sent to Oak with the rest of them. This could imply that Brock simply didn't want the Tauros he caught and decided to copy Ash, perhaps finding the idea funny.
Nonetheless, it should be noted that the ball Brock used to catch the Tauros belonged to Ash, as it was one of the 30 Safari Balls he was given when he entered the Safari Zone. This means that, while Brock threw the ball, the captured Pokémon belongs to Ash. However, nothing would have prevented Ash from transferring ownership of the Tauros to Brock with Professor Oak's help, meaning that either Brock didn't want the Pokémon or Ash simply wanted to keep the herd for himself.
Brock's Tauros is a fascinating bit of Pokémon trivia due to its link to a banned episode and the fact that it raises interesting questions about how the Safari Zone and the Pokémon world as a whole operate. Since the series never touches on the formal laws and rules surrounding Pokémon ownership and the usage of Poké Balls, things like this are left up to fan speculation.