We know that slowpoke evoles in slowbro but mega evoles given to Pokemon when it is last evolved but slowbro has mega evole but it is 2nd. Form how it became mega because it's evole is slowking and mega evole work when it will became 3rd form

slowpoke evolves into slowbro OR slowking, slowbro and slowking are both final forms of slowpoke, slowking was introduced in the second generation and its obtainable in the main games by receiving one from trade while equipped with a king's rock

That hurts to read...