Answer:
Stealthy as a shadow in the dead of night, cunning but affectionate if given a bite. Never owned but often loved. At my sport considered cruel, but that's because you never know me at all.
Thirty white horses on a red hill, first they champ, then they stamp, then they stand still.
It doesn't live within a house, nor does it live without. Most will use it when they come in, and again when they go out.
I move without wings, Between silken string, I leave as you find, My substance behind.
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
What begins and has no end? What is the ending of all that begins?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?