Renting a car in Thailand requires a valid driver’s licence (from your own country) and a contract with the renter. Most of the major international car rental companies like Avis, Hertz, Budget and so on, operate in Thailand, along with many smaller, local ones. Large companies might require a credit card, while some small operators try to hold your passport. It’s not wise to allow that, for Thai law requires that foreign tourists carry their identification at all times.
The smaller, private companies usually charge lower fees, though full insurance is often not included. Third party insurance in required by Thai law, so do ask for and check that any vehicles does carry that minimum, legal coverage. You decide if you want to pay extra for your own insurance coverage.
Operators who don’t have their own business premises are more likely to try pulling a scam. The more serious ones usually involve fake charges that damage was done to the vehicle, followed by claims for huge damages. A minor scammer may demand a full tank of petrol on return, when it was not full in the beginning. Renting from an operator with fixed premises is always safer.
With motorcycle rentals it is much the same, though most owners do not ask for a licence. Tourists without a valid motorcycle licence from somewhere risk getting pulled over and fined by the police. Be sure to wear helmets at all times, for the police love to catch tourists without helmets and collect fines. Sometimes the police check only helmets, letting tourists pass if they just have a licence for a car.