Texas draws a line between mopeds, scooters and motorcycles, and the thresholds have moved over the years. Rather than publish a rule that may have changed since, here is how to work out which category a machine falls into - and then confirm it with Texas DPS, whose rules these are.
What decides the category
Three things generally matter: engine displacement, the machine's top speed, and under some definitions whether it has pedals. A 49cc machine and a 150cc machine are not treated the same way, and a 300cc scooter is treated as a motorcycle for most purposes.
What to do about it
Tell us the exact model you are considering and we will go through registration, plates, insurance and which licence class applies to that specific machine before you buy it. We do this several times a week. If you would rather read the regulations yourself, Texas DPS publishes them.
A note on insurance
Liability coverage is required on anything registered as a motor vehicle in Texas, and it is inexpensive on a small machine. We can point you at agents who write powersports policies rather than treating a scooter as an oddity.