Ending the Year With a Difficult Riddle

An iron horse with a flaxen tail.
The faster the horse runs,
the shorter his tail becomes.

What is it?

(HINT: Think Small)

Riddle, of course

A house with two occupants, sometimes one, rarely three.
Break the walls, eat the boarders, then throw away me.

What am I?

Try This Riddle

Some say I'm their worst enemy,
Some say I'm their best friend.
Some use me in brevity,
You'll still pay for what I lend.

There's proof that I can even kill,
Too many take that path.
There's claim also that I can heal,
You'll have to do the math.

Yes, uses for me do abound,
You'll see me often, look around.

What am I?

Who Will Solve the Riddle?

Before I came, confusion abounded.
I'm late, I'm late was frequently sounded.

I'm not average, but was based on a mean.
My size, in theory, is constant: fifteen.

I'm two dozen steps, again in theory.
But walk my length and you'd get weary.

I take half and quarter steps at times.
In reality, I don't follow the lines.

I shrink to nothing in two cold extremes.
Over a thousand miles wide in the betweens.

What am I?