Can someone explain that theory of Luffy turning into a monkey or great ape or whatever for his next form?
Yes, this is a legit theory I've heard mentioned a couple times from long time fans and not one that I've made up so don't be like:
"Ah! these damn noobs know nothing and come with their piece of trash, good for nothing, hopelessly stupid, dumb, retarded, cancerous, useless, low IQ, shit, worthless, moose caca, crap, worse than Alzheimer's, garbage theories that make them seem like they have the intelligence of a gnat and we should wipe them off the face of the Earth with the force of a thousand suns!!!" Or whatever L'olonais Zoro said about my comment earlier.