The Marty Robbins song Strawberry Roan describes the story of a cowboy trying to break an old horse, the eponymous Strawberry Roan (I'm unsure of any connection to the various films of the same name).

There are three verses describing the horse (in negative terms emphasising his age and poor physical condition), and the attempt to ride it (emphasising his wild and untamed temperament), with the middle of the three acting as a transition between the two.

Down in the horse corral standin' alone
Is an old caballo, a strawberry roan
His legs are all spavined, he's got pigeon toes
Little pig eyes and a big roman nose
Little pin ears that touched at the tip
A big 44 brand was on his left hip
U-necked and old, with a long, lower jaw
I could see with one eye, he's a regular outlaw

I gets the blinds on 'im and it sure is a fright
Next comes the saddle and I screws it down tight
Then I steps on 'im and I raises the blinds
Get outta the way boys, he's gonna unwind
He sure is a frog-walker, he heaves a big sigh
He only lacks wings, for to be on the fly
He turns his old belly right up to the sun
He sure is a sun-fishin', son-of-a-gun

He's about the worst bucker I've seen on the range
He'll turn on a nickel and give you some change
He hits on all fours and goes up on high
Leaves me a spinnin' up there in the sky
I turns over twice and I comes back to earth
I lights in a cussin' the day of his birth
I know there are ponies that I cannot ride
There's some of them left, they haven't all died

(Lyrics from LyricFind via google)

I understand, or have been able to look up most of the terms, but the phrase "frog-walker" in the middle verse eludes me. Seeing as it occurs right at the transition, I'm not even entirely whether it's some sort of poor aged gait, or a reference to strong bucking (likening it to a frog jumping). The fact it occurs before a big sigh makes mean lean on the aged angle, but that the immediate next line talks about how near he is to being able to fly pulls me in the other direction.

Is this a known term describing horses (especially during rodeo or initial breaking), and how should it be understood?