what was the mote of mark's brythonic name

The name is Mark ; the site “Mote of Mark” is named after Mark, King of Dumnonia.

What that means

• The monument’s modern name comes from the figure Mark rather than a separately recorded Brythonic form.

• I did not find a source in the available material giving a different Brythonic version of the name.

Brief context

• “Mote of Mark” refers to an ancient fort site in Dumfries and Galloway.

• In the material available here, the naming is explained simply as deriving from Mark, King of Dumnonia.

TL;DR

• The Brythonic name you’re asking about appears to be Mark itself, with no alternate Brythonic form stated in the sources I could verify.