It’s supposed to be a 50% chance that Delta Muk will have Regurgitation when it Evolves. However, I have reset multiple times and it always has Sap Sipper. Am I just unlucky, or am I stuck with Sap Sipper?
You always get the Sap Sipper ability on Delta Muk when it evolves, you can change it to Regurgitation with an ability capsule, they are sold in a hidden house in the Murk Forest.
so basically you can have ability 1 or 2 or 3 (hidden)
when u encounter a mon the game generates a 1 or 2 since delta grimer has only 1 ability it means that u can only see its ability after it evolves