Three orangutans were seized in Chumphon's Muang district on Wednesday while being delivered to Bangkok.
They were found in a pickup truck driven by Winai, 63, with Kalaya, 45, sitting in the front passenger seat. In both cases, the people's surnames were not given.
The animals, kept in large baskets, are endangered species under the Convention on International Trade in Endangered Species (Cites).
The truck was flagged down for a search by police at a checkpoint near a petrol station.
The police worked with the US Fish & Wildlife Service to mount an investigation, which led to the seizure of the orangutans.
The officers also discovered in the truck what they believed to be wild monkeys and a goral, a small goat-like mammal.
The animals may have been smuggled into the country by an international wildlife trafficking network, police say.
Mr Winai and Ms Kalaya told police they had picked up the animals in Songkhla and were delivering them to a customer in Bangkok.
Police were checking to see if a licence had been obtained to possess any of the animals or if permission had been sought with the authorities to own them.
Experts at the Department of National Parks, Wildlife and Plant Conservation were running a check to determine where the animals might have come from.
At the same time, police expanded their investigation and raided a house linked to the two suspects in the Ratchapruek area of Bangkok, where they found seven cotton-top tamarins and two red pandas.
In December 2023 the government sent three orangutans, which had been trafficked to Thailand seven years prior, back to Indonesia.
An orangutan is transported to an environment protection office after it and two others were seized in Chumphon while being delivered to someone in Bangkok on Wednesday. Department of National Parks, Wildlife and Plant Conservation