Make a mega evolution for any Pokemon you want. Try to balance it for OU. Alternatively, just have fun and break it completely. Entirely up to you.
Rate other people's megas and call eachother faggots. Fun times all around.

>Mega Tentacruel
The general idea behind this Tentacruel was to give it some form of recovery. The be specific, Rest + Natural Cure means it can come in repeatedly throughout the match to use Rapid Spin, throw out Scalds, set Toxic Spikes, or do whatever. Then when it wants to leave, it can use Rest and switch out so it's healthy the next time it comes in. Good speed and SpA means it can run an offensive set (or at least coverage) if it wants to.