Witnesses said a passenger at the back of the plane reportedly saw a scorpion crawl out from a sleeping seatmate's sleeve and run under the man's chair.
Passengers initially reported a man had been stung, but United said there were no injuries.
The airline issued a statement in response to the incident:
"Houston to Quito flight 1035 returned to the gate after a scorpion reportedly emerged from a customer's clothing. Paramedics at the gate immediately examined the customer and determined that he had not been stung. The customer declined further medical treatment, and as a precaution, a new aircraft was arranged. We provided all passengers with a meal voucher due to the delay and the flight, with the customer aboard, has departed for Quito."
It was unclear whether the scorpion was ever located.
Passengers said the flight departed three and a half hours late.
The incident comes a little less than a month after a man was stung by a scorpion on a United flight from Houston to Calgary.
