William: 'He can't hurt me. Let him
  listen. It's a horse apiece, for I'm the
  King of China.' When I saw the Prince
  of Wales I asked him for a chew of tobacco, and kicked because it wasn't the
  brand I was used to chewing. He tried
  to get back at me by saying no man of
  my nationality and name could talk
  back to a peer of the realm. I said to
  him: 'Come off de perch, you want
  do see?'

For some background, the article says 'Workhouse' Kelly was a nearly 60-year-old Irishman who had lived in St. Paul for the previous 20 years, spending 90 days to six months of each year in the workhouse. He hadbeen a sailor in the US navy in the 1850s and was "full of yarns".
The other two uses I found in Chronicling America are both from the Rock Island (Illinois) Argus. First from May 31, 1899 describing two baseball teams as appearing equal before a match:

It's a horse apiece. The shake-off occurs today.

Second from April 18, 1904, also describing two baseball teams:

It's a horse apiece now with the Rock Island and Davenport on the ante-season baseball games. Rock Island won
  yesterday afternoon's exhibition game
  at Twelfth street park 4 to 3, the same
  score by which Davenport defeated
  the local aggregation on the grounds
  across the river a week previous.

The Word Detective, after confirming the phrase isn't "it's a horse of peas", said