The Indigo Disk, Pokemon Scarlet and Violet's second DLC, has introduced a new evolution for Dipplin, the evolution of Applin. The new evolution is Hydrapple, and you will need to advance through the first few story quests of the new DLC to get access to their evolution. Applin cannot be caught in the Terarium in The Indigo Disk, you will need to catch Applin and evolve them into Dipplin in Kitakami--the area from the first DLC--using a Syrupy Apple. Once you have a Dipplin, here's what you need to do to evolve them into Hydrapple.
How to evolve Dipplin into Hydrapple
To evolve Dipplin into Hydrapple you will need to teach them the move Dragon Cheer and level it up. The move Dragon Cheer will need to be taught using TM 226, which is awarded for beating Drayton of the Blueberry Academy Elite Four. Drayton is the dragon-type user located in the Polar Biome. To beat him you will need to take down his trial, which requires you to beat three trainers using only Pokemon caught in the Terarium. After that, you have to beat Drayton in battle. Upon beating him, you will unlock TM 226, Dragon Cheer, which can be crafted at the TM maker in the Central Plaza. If you don't have access to the elite four battles, you need to progress further in the main story.
Drayton will also give you a Dragon Cheer TM to use, but if you need to craft another one it requires 1,500 League Points, along with 2 Tatsugiri Scales, 2 Lapras Teardrops, and 1 Applin Juice, all of which can be obtained by defeating those Pokemon in the wild. After teaching Dipplin Dragon Cheer, you need to level it up once, either in battle or by using EXP candy, and they will evolve into Hydrapple.