சேவகன் is the best way of expressing the word "bot". The actual meaning of bot is a program or robot which is helpful to users. சேவகன் is also kind of polite word which helps to users. கூளி is an ancient word that is not known to may users and also not a polite word

I must appreciate your interest in this word. At the same time, கூளி is 100% gender neutral, but சேவகன் mostly considered as male and it's a borrowed word from northern language. Moreover, pronunciation of word கூளி is similar to very familiar word "கூலி" (coolie) which means hard worker. Here, bots are doing hardwork like ghost. Hence, the ancient word கூளி, the ghost warrior to complete any task, is used.