Origin of the Thai letter ฑ (tho montho) How did the character Montho from Ramayana make it into the name of the Thai letter ฑ (tho montho) of the Thai alphabet. Where is the relationship and in what year was this connection established?
Thai is one of the most complicated written languages in the world..  (not saying most efficient).  One of the rules is if a word is from foreign language, a new alphabet and a written structure is invented so that it would be distinguishable between original thai and foreign languages, which mainly Pali or Sanskrit.  

To give an example of a word "nirvana"  since it is not English, there would be a need to invent a new letter that produces "n" sound and new structure on how to put letters together so the goal is to communicate to readers that "nirvana" isnt English origin.  Lets say "&amp;^" is a new "n" sound, nirvana would spell "&amp;^ervanah*".  For that, now "n" sounds has two representations in Thai Language "n" and "&amp;^"  and every time "&amp;^" is used it cant be used with "i"  etc.   Most of foreign languages in thailand at the time of beginning of written system were Pali and Sanskit around 700-1000 years ago.  So my guess is, ฑ (tho montho) is younger than 1000 years old.
I dont think religious beliefs were not as strong driving force as the idea of separating foreign languages to original Thai. 

tho montho produces "T" sounds but not of Thai Origin.  If word is originally Thai, "t" sound, would use "tho thahan ท"

this guy put a video explaining thai written system.
thai written language explained

PS. Dont try to pronounce island of Phuket in Thailand, with an F.