Melanie, these tartans are historical in that they are officially registered with the Scottish Register of Tartans, and listed on their official, government website, with a URL ending in .gov.uk
I know from experience that you can register a tartan of your own design with them, for a fee. Some people, such as Domino's Pizza retain copyrights to their official tartan threadcount, and they & the Scottish Register refuse to divulge it. Some tartans are so ancient that it is no longer known when they were invented. The Swedish Tartan is dated 21 Oct. 2005, with this note:
"For Swedish people worldwide, and those associated with Sweden. Approved by Torvald Colliander, Consul General in Edinburgh. For further information please visit www.internationaltartans.co.uk"