As with all good song lyrics, they can be interpreted differently. Here's some extracts of the discussion at Cowboy Poetry at the Bar-D Ranch:
snuffy meant "snuff colored, or reddish-brown."
fiery and snuffy referred to two different colored dogies and I think I remember my teacher telling me that it referred to the names of two locomotives waiting to take the cattle to market after the roundup.
In the Songs of the Wild West, with commentary by Alan Axelrod, from the Metropolitan Museum of Art 1991, the author says "...fiery (another term for paint) and the snuffy (a buff- or snuff-colored horse)..."
Regarding the meaning of "the fiery & snuffy" have always meant (to me) that the ones prone to spooking & snorting (the fiery & the snuff-y) are just LOOKING for an excuse to stampede. Seems perfectly obvious! And therefore, you would want to "ride around them SLOW."
The "Fiery & Snuffy" in "I Ride and Old Paint" refer to Lighting and Thunder. Those names for it were well known to anyone in Wyoming and Montana at the time the song was written. Note that the line is: "For the fiery and snuffy are ready to go." Often it is sung as: "For they're fiery and snuffy and ready to go," which also works if you consider the cattle to be fiery and snuffy. What it really refers to, though, is lightning and thunder.
[My dad] said "fiery" and "snuffy" were the campfire and branding iron.
Having worked as a cowboy in Montana, Wyoming, Nevada, and Idaho, and about the terms "fiery and snuffys": whenever you're holding up a herd of cattle to work, be it branding or cutting out steers, there are always a certain few in the herd that are looking for a chance to breakout of the hold up and head back to their home range.
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