This complex Celtic art "Carpet Page" is a bit like the full page decorative panels found in illuminated manuscripts like the Book of Kells or the Book of Lindisfarne.
It's been a popular image that often appears, unattributed, on the web. It's available on posters, archival prints, and cards; a slightly different version has been adapted as a jigsaw puzzle from the Wentworth Wooden Puzzle Company in the United Kingdom.
"Sun Horse, Moon Horse" is one of Bradley's most popular designs, either as a poster or, as offered elsewhere, as a t-shirt. Two opposing horses - one gold, for the sun, one silver, for the moon - face off in a Celtic knotwork panel that unites them in a single pattern.
One of two Pentacle designs, this golden one, like its silver counterpart, is itself a Celtic knotwork pattern - which is then framed and ringed by other Celtic designs, and by a total of eighteen knotwork dragons.
It's available on a number of products - it's even been turned into an embossed leather journal - but t-shirts with this design aren't available here. You can find them at The Celtic Art Works.