Do you need a co signer for a car with monthly payments?

That IS a car loan. Needing cosigner depends if you have good enough credit.

Monthly payments says there is a loan.
Whoever is allowing you to make these monthly payments will decide whether they trust you alone to make the payments, or else that they don't trust you and you need to provide someone who promises to make them if you don't.

Monthly payments IS a car loan. Geez, how did people get so stupid?

The only way for this to happen is if you go to a Buy Here Pay Here dealer who does "in-house financing". They will charge you insane interest rates on an overpriced used car.

A reputable dealer will sell you a vehicle, but unless you pay cash, it will be financed. When you finance a vehicle, you and the dealer come to an agreement on the price, plus tax, title, and license. A lender will pay the agreed upon price, in essence buying the vehicle from the dealer. You will, in turn, pay the lender for the vehicle.

When you make monthly payments it IS a car loan! Unless you have a good paying steady job and good credit history, you will need a co signer.