Use of 龙 in the idiom 车水马龙
I quote some words from my dictionary specified for idiom(成语)


  成语，形容车像流水，马像游龙。形容来往车马很多，连续不断的热闹情景。 


Let me try to translate 


  idiom,it describe the scenery that the wagon and the horse come and go just like the stream and the dragon.
  
  马--->horse,龙--->dragon,they have no relationships with each other.In Chinese language,we often use metaphor ,here,we use dragon(龙) to describe the busy horse（马）.


There is no word called 马龙 in Chinese,but it is of great possibility to have someone called 马龙,because this year is the year of dragon,if someone has the family name 马，then his parents may call him 马龙。

there is another idiom 龙马精神，it describe  one man with great passion.