Little Johhny England do a nice song called 'Early to Bed' about plough horses:

"Early in the morning
In the pitch black and the cold
I lead the team across the Yard
And wait as I've been told........"

And, though it's not strictly folk, this is a classic:

HEAVY HORSES (Ian Anderson)

Iron-clad feather-feet pounding the dust,
An October's day, towards evening,
Sweat embossed veins standing proud to the plough,
Salt on a deep chest seasoning.

Last of the line at an honest day's toil,
Turning the deep sod under,
Flint at the fetlock, chasing the bone,
Flies at the nostrils plunder.

The Suffolk, the Clydesdale, the Percheron Vie
with the Shire on his feathers floating.
Hauling soft timber into the dusk
to bed on a warm straw coating.

Heavy Horses, move the land under me.
Behind the plough gliding slipping and sliding free.
Now you're down to the few
And there's no work to do:
The tractor's on its way.

Let me find you a filly for your proud stallion seed
to keep the old line going.
And we'll stand you abreast at the back of the wood
behind the young trees growing.
To hide you from eyes that mock at your girth,
and your eighteen hands at the shoulder.
And one day when the oil barons have all dripped dry
and the nights are seen to draw colder
they'll beg for your strength, your gentle power
your noble grace and your bearing.
And you'll strain once again to the sound of the gulls
in the wake of the deep plough, sharing.

Standing like tanks on the brow of the hill
Up into the cold wind facing
In stiff battle harness, chained to the world
Against the low sun racing
Bring me a wheel of oaken wood
A rein of polished leather
A Heavy Horse and a tumbling sky
Brewing heavy weather.

Bring a song for the evening
Clean brass to flash the dawn
across these acres glistening
like dew on a carpet lawn
In these dark towns folk lie sleeping
as the heavy horses thunder by
to wake the dying city
with the living horseman's cry

At once the old hands quicken
bring pick and wisp and curry comb
thrill to the sound of all
the heavy horses coming home.