The Icelandic Horse competition rules allow for 10 oz weighted boots....

.... for protection.

Why would they need that much weight for protection?

Could it be for gait enhancement?

The National Bridle Shop, which caters to show tack for Tennessee Walking Horses, makes a separation between training boots and protective boots:

Protective Boots

Training Boots

As you will see, even the Tennessee Walker industry classifies the boots that are used by Icelandic Horses for *protection* as weighted training boots.