Here's one early African-American source from 1911

Lyr ADD: MULE-SONG

From JOAFL Vol. 24 1911 Article by Howard Odum:
"The negroes have much to say about the mule in their work, and
have much to do with him in actual life. Their songs also contain
references to him. A mixture of parts of song added to experience
and imagination produced the following "mule-song:"

I went up Zion Hill this mornin' on a wagon,
I went on a wagon up Zion's Hill this mornin',
The durn ole mule stop right still this mornin', this mornin', so soon.

I got out an' went 'round to his head this mornin',
I got out an' went 'round to his head this mornin',
The durn ole mule was standin' there dead, this mornin', so soon.

Yes, I hollow at the mule, an' the mule would not gee, this mornin',
Yes, I hollow at the mule, an' the mule would not gee,
An' I hit him across the head with the single-tree, so soon.