what am I?

it carries much weight
but not of the world
but of it's scaffold
to the glimpsing of worlds

it's nails are not bent
it's foundation not beauty
to forward a purpose
in hurry it's duty

holds paint
at low hight
with seldom seen
cracks
with the killing of dogs
and runs from its tracks

it runs past it's brother
trapped in a box
to showcase it's house
of plastic and leather

it moves to new houses
with changing of weather
and into a mouth you dare never ever.