You're welcome, Guest.   

Joe Offer- I think this is an accurate transcription now; note that line 5 has been altered from "I've swung to many's a saddle sing" to "I've swung to many's the saddle since", "Garry Goulach" is now spelled correctly, and the title of the poem is properly "The Black Horse".
**********************
Corrected version:

The Black Horse by Archie Fisher

When I rode the grey horse that carried Merlin's name
I broke the wizard's curse to make the bold moss-troopers claim
Again I rode a lean grey in the mountains of the west
Through hanging bog and corry to Garry Goulach's crest
I've swung to many's the saddle since on bay and chestnut too
But I will ride the black horse 'til I ride home to you

And I will ride the black horse with a lurcher at his heel
From Lammermuir to Cheviot and along by Teviotdale
Across the windy Minchmoor and down through Liddesdale
And o'er the crag of Ruberslaw to Bowmont and to Kale
For I will take the long track where every day is new
And I will ride the black horse 'til I ride home to you

And I will ride the black horse all the summer long
To hear a distant echo join the chorus of a song
And when I cross the Ettrick or ford the Yarrow's flood
In the autumn woods of Tweeddale I'll feel you in my blood
And as the winter westerlies come howling through Traquair
I'll turn his head into the wind to taste you in the air
And when spring creeps o'er Soutra Hill and nature loves anew
Then I will mount the white horse and I'll ride home to you.