Answer:
Different lights do make me strange, thus into different sizes I will change.
I'm not man's best friend, I'm their enemy, I can mark your end, yet you do not see me, I am very small, but very tough, If you have me, then you have it rough.
I can travel from there to here by disappearing, and here to there by reappearing.
What kinds of men are always above board?
Talks like this, a green wise man does. Guess my clue, you will.
People take me on walks and I run and have fun. What am I?