Answer:
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.
A seed am I, three letters make my name. Take away two and I still sound the same.
Come up and let us go. Go down and here we stay.
Three little letters, a paradox to some. The worse that it is, the better it becomes.
Different lights do make me strange, thus into different sizes I will change
When is it bad luck to see a black cat?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?