Sionnach (pronounced “Sho-nock”) is Irish for “fox.” The Sionnach specifically refers to the Seidr Sionnach who became legend.
“The Seidr Sionnach,” Kallan repeated. “The Sionnach were a pair of foxes twice as tall as a man and strong enough to pull a sleigh across the snows of Jotunheim.” She stared at the lights of Odinn’s Valkyrjur. “They loved each other beyond this world until one’s release was the other’s breath. They roamed wild and free, wreaking havoc, nonsense, and all sorts of mischief among Men. All who looked upon the Sionnach feared them and so…despised them.”
“One day,” Kallan continued, “Freyja looked down from Asgard and saw the Sionnach frolicking without worry of what Men thought. Their adulation became her conviction. And she loved them. She loved them so much that she gave them her Seidr. Freyja taught them how to wield it, and the Sionnach grew in power and strength alongside her. Over time, they too grew to love her. Some stories tell of how they pull her chariot across the skies.” Kallan grinned. “It was said that from the earth, the Sionnach, with their red Seidr flames trailing behind them, looked like two red cats bearing Freyja across the night.”
– Excerpt from Chapter 60 of Dolor and Shadow