The situation you're in illustrates an essential point: never -- but never -- enter into an arrangement of the type you have until you have in advance worked out and agreed on the terms that will apply should certain outcomes be achieved. I'm not saying this to be captious, but because I see this time-and-time again. Agreements should anticipate all reasonable -- and even unreasonable -- outcomes; and then specify what will happen if and when those outcomes occur. This is how lawsuits are avoided!
Now, in your situation, this is somewhat complicated. The President approved it, but with what understanding, if any? Depending on the state you live in, you may otherwise have rights if it's unrelated to the company's business, but that depends on your employment agreement (yes, it's really worth reading this and negotiating them!). But as, Clive rightly points out, if you've done this on company time, then any rights you may have otherwise had likely revert to your employer unless you have an express exception codified in writing and signed by the relevant parties.
If your President has, as Clive says, an abundance mentality, then you may be able to clarify and codify additional benefits to yourself. But even if the President doesn't, that doesn't mean you have no possibilities here. For example: are your skills needed going forward and in a significant way? That's negotiating leverage. Will the project fall apart if you're not there? An abundance mentality isn't needed for you such leverage if you're a critical success factor. If that's not the case, then your leverage is weak. Renegotiating is muchharder than negotiating properly and fairly in the first place.
Last, you need to look at this objectively. Perhaps the deal you have now isfair. You did all the work, but how much work, how much brilliant ideation? You won't be selling it. Will you be marketing it? If you invent something and another company manufacturers, distributes, markets, and sells the product, then 5% isn't unreasonable. So perhaps your single-digit equity share is fair. Good negotiators are looking for something that is fair and equitable to both parties, that avoids litigation in the future. In some cases, a 1% share is fair; in others, it's 80%. It all depends. So before you even consider renegotiating, you need to determine if you've already been treated more than fairly, and if so, you should be ecstatic. If not, then you can apply some of the points I've made above. Good luck to you.