From ErfWiki

Speculation

Nickelhorses (spelled both "nickel horse"^{IPTSF Text 3} and "nickelhorse"^{IPTSF Text 5}) are Mount Units in service to Goodminton - together with Quarterhorses.^{IPTSF Text 3} Their appearence and abilities are yet unknown.

Nickelhorses are used as mounts for Goodminton's knights. Warlord Rex Cakes rides nickelback too.^{IPTSF Text 18}

Real World References

Together with quarterhorse, their names may be references to 25-cent (quarter) and 5-cent (nickel) coins.

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.