what's the meaning of idiomatic expressions like "man still riding on a white horse" & "bites the dust"?

Answer 1

Think about the expression and it will give some help. To "bite the dust" you have to be lying face down on the ground - in most cases you would have to be dead to be in this position - bite the dust means to die.
The other expression could be a miss quote. It sounds like it could be "riding a high horse". This means an attitude of looking down on everyone from a moral point of view - criticising others for their actions and beliefs.
Remember,as they are "idiomatic" they won't be the same every where. In Australia we have the term "a country mile"- an expression that has no meaning in small densely populated areas.