There is no way of knowing specifically the first tartan ever created, given how long they have existed. However, with certain magical online tools, we are able to at least see more modern renditions, who designed it and when.
A Tartan is a design by which 2 or more colours are set to a pattern and weaved to create an accumulation of lines and boxes. Certain mills may have different coloured threads, meaning it is important that you are aware of the specific colours of yarn.
In the past, tartans were worn mainly by highlanders. During the 18th century when Tartan was associated with government rebellion, it was adopted by lowlanders as a general statement being made.
It began to grow within Scottish culture, with more and more tartans being designed and registered. According to the Scottish Tartan Society (STS), Scottish Tartan Authority (STA) and the Scottish Tartan World Register (STWR) there are 8165 tartans on the register, made up of legacy tartans formerly recorded by such authorities. On top of this, there are over 2400 tartans registered on the Scottish Register of Tartans since its implementation on February 5th, 2009.
Any new tartans created are registered with the Scottish Register of Tartan, where a huge catalogue of tartans can be found and the rights to be woven can be acquired from specific tartan Designers. This is useful if one day you wish for your own special weave or custom tartan is being made.