Equity entitlement needs to be something you can legally enforce, if needed, so it needs to be in writing in some manner. If it's not in the employment contract itself, there needs to be something else.
A couple of years ago, I joined a startup fairly early on, and they were still figuring out lots of legal details. So my offer letter remarked on what percentage of the company I'd get options for, and said a formal options agreement would come later - which it did, after a few months. That was fine with me - if I'd needed to litigate, that offer letter would have been adequate.
But if all you have is a verbal promise, unless other people were present and willing to testify to it in court, that promise is pretty worthless.
If you are hiring an employee, and offering them equity, that must definitely be in the contract.
Another question is whether you understand how to, or are able to split equity. There are many legal complexities if you want to get that right.