Pietra's grandmother sprained her ankle and Pietra wanted to make it easier for her to travel through the mountain by digging a tunnel through it. She was using Ground-typePokémon to try to achieve this. As they dug deeper, water began to drip onto them, which caused the Ground-types to be scared off. Hence she resolved to capture a certain Rhydon which was able to use Surf and as such, wasn't afraid of water.
Pietra met Ash and his friends, and together they came across Rhydon. It escaped into the lake, however. Ash and Misty used their Water Pokémon to help search for it. Pietra used her speedboat to go out onto the lake. Soon, Totodile discovered Rhydon by accident and bit on its horn, angering it. Pietra tried to battle it, but was unsuccessful after it used Horn Drill in the water and caused a whirlpool.
They then traveled over to a small island where they saw Rhydon coming to shore. Pietra encountered it again, but Team Rocket interfered and tried to take it for themselves. Ash assisted in stopping them and the Rhydon jumped back into the lake. Pietra jumped into the lake after it and latched onto it. Rhydon carried her under water, but she refused to let go until it battled her, and eventually the Rhydon finally gave in and agreed.
She used her Marill to battle and defeat Rhydon, which allowed her to catch it. She then used her newly caught Rhydon to complete the tunnel.
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Marill is Pietra's main Pokémon. It is her only Pokémon that is not a Ground-type, and Pietra often carries Marill around in her arms. She used it to battle a Rhydon that knew Surf. Marill was able to weaken Rhydon enough so Pietra could capture it.
Pietra wanted this Rhydon because of its ability to Surf. She needed a surfing Pokémon because her grandmother sprained her ankle and Pietra wanted to make it easier for her to travel by digging a tunnel through a mountain. Unlike most of her Ground-type Pokémon, Rhydon wasn't afraid of water. It is considered to be very strong as it knocked out many Pokémon that tried to battle it.
It proved to be quite a reclusive wild Pokémon, and would only attacks others when threatened. It was shown to be particularly fond of apples, and this helped Pietra eventually track it down.
After witnessing Pietra's determination, Rhydon eventually conceded and agreed to battle against Pietra. Rhydon went up against Pietra's Marill, and it was able to use its Horn Drill to block any otherwise effective Water attacks. However, an Iron Tail allowed Pietra to capture Rhydon on her second attempt. It later helped Pietra to complete her tunneling project.