G'day,
We were at a poetry night last week and sang the above song. Afterwards one of the poets said he thought Tennyson had written a poem telling this story, and that the hound's name was Gelert. A search of the net failed to find a relevant Tennyson poem, but a search for gelert found these.
http://www.nwi.co.uk/beddgelert/legend/
http://www.g-net.demon.co.uk/gelert.htmCheers,
Alan