what are fay horses ridden by the sidh called in celtic mythology

The Fae horses ridden by the Sidhe in Celtic mythology are usually called the Sidhe’s horses or, more broadly, the fairy host’s steeds ; in Irish tradition, the mounted fairy procession is often associated with the Sluagh Sídhe or “fairy host.”

What the term means

The word Sidhe refers to the fairy people in Celtic tradition, especially the supernatural beings linked with hills, mounds, and the Otherworld.

When folklore describes them riding at night or during Samhain, they are often pictured as a procession of fairies on horses rather than a single named breed of magical horse.

Useful nuance

If you were thinking of a specific mythic horse name, Celtic folklore is inconsistent here: the procession is named more often than the horses themselves.

So the safest answer is that they are usually described as the fairy horses of the Sidhe or the horses of the Sluagh Sídhe.

TL;DR

They’re generally called the horses of the Sidhe or the Sluagh Sídhe’s horses , not a single fixed named species.