[cgbg]

Folks here have figured it out...a Pony Soldier was an Indian who ratted out (and/or hunted down) other Indians for the Feds.

Dog-faced means ugly.

So, a dog-faced pony soldier is an ugly traitor.

[jmacusa]

“Pony Soldier’’ was a Indian epithet for US cavalry troops. Someone who couldn't manage to tame and ride a horse would be relegated to riding a pony. It means, in a sense, a person is weak and insignificant.