Mod 1.12.2 Nether Knight adds new entities. The Nether Knight is a mini boss who occasionally appears in Nether biomes. The four shields they have provide protection against many types of attacks.
These knights can summon reinforcements when they are hurt. When they block an attack, their shields can sometimes cause fire damage. The knight can drop Blaze Rods if they are defeated. There is also the possibility that they might drop one of your shields.