Cataphract
1. Defensive armour used for the whole body and often for the horse, also, especially the linked mail or scale armour of some eastern nations.
2. A horseman covered with a cataphract.
3. The armour or plate covering some fishes.

The word is derived from the Latin cataphractes, Greek καταϕράκτης, a coat of mail; also from Latin cataphractus, Greek κατάϕρακτος, clad in full armour.
Surprisingly the OED gives the first recorded usage in 1581.