Nickelhorses (spelled both "nickel horse"IPTSF Text 3 and "nickelhorse"IPTSF Text 5) are Mount Units used by Goodminton. Along with Quarterhorses, they are ridden by Knights. Warlord Rex Cakes rides nickelback, too.IPTSF Text 18
Nickelhorses are likely made of US 5-cent nickels, or something similar. Alternatively, they may consist of the metal nickel.
They might be a form of Golem, potentially being made by a Moneymancer. If so, their presence in Goodminton would suggest they may have had a Moneymancer at one time, or hired one, or that one or more of their cities could produce them (which would lend credence to the idea that cities can produce golems without a caster of the relevant discipline on their side).
Real World References
In Goodminton, Wanda observed how "stalwart knights led alternating charges at one another on quarterback, then on nickelback". This may be a reference to quarterback and nickelback - player positions in American football.
Nickelback is a Canadian hard rock band.
Nickel is a chemical element, specifically a metal.