I wonder how and where “they” asked you for that kind of money. The Thai government customs official importation of animals (dogs and cats fall into the category of livestock) into Thailand clearly stated that the import fee is 500B (I imported a dog and a cat hence 1000B) . I paid this amount inside customs with a receipt, all officially done. Did they pull you over to a corner to ask for the money?
I am sorry but what a horrible comment. I brought my dog from Hong Kong to Bangkok Suvarnabhumi Airport 6 months ago and it was all smooth. Prepare all my documents according to the Customs website. The Quarantine officers checked all documents, scanned for microchip then it was all done, then we proceeded to customs to pay a fee think it was a 1000B, got a receipt and walked out of customs. My dog was even allowed to walk inside the restricted area (where you get your luggages) and all the way to the car pick up.
BTW the quarantine department is open 24 hours but do inform them in advance your expected arrival time.