The Arms
Blazon

Shield: Sable, a lion rampant regardant Or.

Seisyllwg was a kingdom which emerged from the expansion of the Kingdom of Ceredigion southwards to include Ystrad Tywi, which included Gower.

The arms of Seisyllwg are "attributed" : they were assigned long after the kingdom's existence. Heraldry did not exist when Seisyllwg did.