Muay Thai was developed as a means of unarmed combat when a soldier’s swords or other weapons were damaged or lost in combat. It was refined into a separate martial art being tested in battle during various conflicts between Siam (Thailand) and the Burmese which was ongoing during that time period. This was about 1560 AD during the rule of King Nareusan. It soon became a popular sport.
During the reign of King Rama VI, Thai boxing matches became more widespread. Matches that used to be held in make-shift rings in any available courtyard became a standard raised ring surrounded by ropes. The first ring was built in 1921 at Suan Kularp field. Although standard rings were available, boxers still bound their hands with rope.
Various forms of martial arts have long been practiced throughout Southeast Asia. As with the most countries in the region, Thai culture is highly influenced by ancient civilizations within Southeast Asia.

The origins of Muay Thai is unclear. One theory is that it was with the Thai people before immigration to Southeast Asia from China. Another is that it was adopted and modified off of Khmer martial arts when Thai culture was influenced by Khmer culture. A third theory is that a little bit of both the first and second theory occurred. Muay Thai evolved from its ancestor Muay Boran ("ancient boxing"), an unarmed combat used by Siamese soldiers in conjunction with Krabi Krabong, the weapon-based style.

Eventually Muay Boran was divided to:
Muay Korat (Northeast) emphasized strength. A technique like "Throwing Buffalo Punch" was used. It could supposedly defeat a buffalo in one blow.
Muay Lopburi (Center region) emphasized movements. Its strong points were straight and counter punches.
Muay Chaiya (South) emphasized posture and defense, as well as elbows and knees.
Muay Ta Sao (North) emphasized speed, particularly in kicking. Because of its faster speed, it was called also called "Ling Lom"
From those days up until the early part of the 20th century, the fighters, particularly those in provinces, used horsehide strips and later hemp in lieu of gloves. 
Rope binding were used until 1929 when boxing gloves took its place. Earlier at the Lumpini Park, a Filipino boxer gave an international style boxing exhibition with boxing gloves. Soon after, leather gloves and weight classes where adopted and Muay Thai as we recognize it began.

Kickboxing
The term “kickboxing” is a term coined by the Japanese. In the late 1950’s and 1960’s, Japanese businessmen who were in Thailand, attended Muay Tha bouts which were held in various stadiums.  Not truly understanding the art, concluded that their best Karateka black belts could defeat the Thai boxers . Returning to Japan, they set up live on close circuit tv, matches between Muay Thai fighters from Thailnd vs Karate Black Belts of Japan. This was broadcast throughout Japan.
Few of the Black belts made it to the past the 2nd and 3rd rounds. Being punished by the Thais. The Japanese not used to being kicked in the thighs with the shins, clinching and being kneed, and then elbowed, was too much for the Karatekas.
After the loss on national TV, the Japanese had “loss face”. They began to secretly sending fighters to Thailand be trained and also recruited trainers to come to Japan and train fighters.  But, because this art was Thai, They couldn’t show the Japanese people they had to adopt another cultures fighting art. So, they came up with the term “Kickboxing”.
Since that time, the Japanese are now referring to it as Muay Thai. “Kickboxing” is now used for other variations of the art.

TRADITIONS AND RITUALS
One of the most recognized rituals and ceromonies of muay Thai is the “Wai Kru- Rham muay”.
Even before entering the ring, the fighter will bow or “wai” prior to stepping into the ring. The fighters cornerman will also pull the top of rope of the ring down so, that the fighter climbs over the top of the ropes instead of the “western” style of entering between the ropes. This is to insure that the “good” spirits remain in the ring. By going between the ropes, the “good” fighting spirits will “escape” the ring. Prior to the wai kru, the fighter will “seal” the ring. This insures that the “good” spirits stay in during the fight. Some may say it’s to keep the opponent from leaving the ring before the fight! Kind of a “zip-lock”
During the “wai kru”, and prior to the fight, the fighter wears a mongkol or headband. In ancient times the headband had Buddhist prayers and chants rolled into the device. This was for good luck. It is removed prior to the fight. To this day, most, if not all mongkol’s have been blessed by a Buddhist monk. After the “wai kru and Rham muay” the mongkol is removed by the “kru” and a short prayer is said for a good fight.

“Wai Kru” translates into “ paying respect to the teacher”. “Wai” meaning paying respect and “kru” meaning teacher or coach.
The fighter is paying respect not only to his/her trainer or “kru” but, also to all those fighters who have fought in the ring before them. Each gym has a “wai-kru” that is specific to that gym or trainer. Avid fans in Thailand can tell which gym a fighter represents by the wai kru. Rham muay portion is the “fighters” dance. This too, is particular to one’s gym or trainer.

Legendary Heroes
At the time of the fall of the ancient Siam capital of Ayutthaya in 1763, the invading Burmese troops rounded up a group of Thai residents and took them as prisoners. Among them were a large number of Thai boxers, who were taken by the Burmese to the city of Ungwa.
In the Burmese city of Rangoon, the king of the Burmese, Hsinbyushin (known in Thai as "King Mangra"), decided to organize a seven-day, seven-night religious festival in honor of Buddha's relics. The festivities included many forms of entertainment, such as the costume plays called likay, comedies and farces, and sword-fighting matches. At one point, King Mangra wanted to see how Muay Boran would compare to the Burmese art Lethwei. Nai Khanom Tom was selected to fight against the Burmese champion. The boxing ring was set up in front of the throne and Nai Khanom Tom did a traditional Wai Kru pre-fight dance, to pay his respects to the Burmese king, as well as for all the spectators, dancing around his opponent, which amazed and perplexed all the Burmese people. When the fight began, he charged out, using punches, kicks, elbows, and knees, pummeling his opponent until he collapsed.
The referee however stated that the Burmese opponent was too distracted by the Wai Kru, and the knockout was invalid. The King then asked if Nai Khanom Tom if he++ would fight nine other Burmese champions to prove himself. He agreed and fought them all, one after the other with no rest periods in between. His last opponent was a great boxing teacher from Ya Kai City. Nai Khanom Tom mangled him by his kicks and no one else dared to challenge him any further.
King Mangra was so impressed that he remarked, "Every part of the Thai is blessed with venom. Even with his bare hands, he can fell nine or ten opponents. But his Lord was incompetent and lost the country to the enemy. If he would have been any good, there was no way the City of Ayutthaya would ever have fallen."
King Mangra granted Nai Khanom Tom freedom along with either riches or two beautiful Burmese wives. Nai Khanom Tom chose the wives as he said that money was easier to find. He then departed with his wives for Siam. Other variations of this story had him also winning the release of his fellow Thai prisoners. His feat is celebrated every March 17 as "Boxer's Day" or "National Muay Thai Day" in his honor and that of Muay Thai's.

Today, some have wrongly attributed the legend of "Nai Khanom Tom" to King Naresuan, who was once taken by the Burmese. However, Nai Khanom Tom and King Naresuan were almost two centuries apart.