But to build a prototype you probably are going to have co-founders who are going to build said prototype with you.

Someone(s) is going to have to throw in a little seed money if only to pay for hosting of the prototype.

And between you and your co-founders you would be ill-advised to begin building that astonishing core value and creating out-of-pocket expenses without some kind of legal agreement in place of ownership and ownership %ages.

That's usually what a corporate entity and cap table are for.

I've seen former best friends and college roommates sue the fuck out of each other because they didn't do this back when they began building said prototype.