The T Dmon Data Pack adds the Stunfisk family, and the two forms spawn in very different places. Normal Stunfisk lives in Swamp biomes. The flat Galarian version lives in Taiga forests. If you only search one biome, you will only ever find one form.
Finding Normal Stunfisk in Swamps
1. Press F3 and check the biome line on the debug screen. It should say you are in a swamp.
2. If you cannot find a swamp, use the vanilla command /locate biome minecraft:swamp (cheats must be on). Then travel to the coordinates it gives you.
3. Normal Stunfisk swim in the wet parts of the swamp. Walk along the shallow water and look closely, because a flat creature is easy to miss among lily pads and roots.
4. Catch it the same way you catch anything else in Cobblemon.
Finding Galarian Stunfisk in Taiga
1. Head to a Taiga biome. You can use /locate biome minecraft:taiga if needed.
2. Search the forest floor between the spruce trees. Galarian Stunfisk is the flat version, so watch the ground carefully.
3. Check the F3 screen to confirm you are still in taiga. The edges of a forest can drift into a neighbouring biome.
Tip: Every creature in this pack has custom animations, so a Stunfisk will move on its own. If something flat on the ground starts shifting, go take a closer look.