Tegan's original pronunciation was "Teh-guhn". The "English" pronunciation with the sound of the word "tea" was originally only an Australian thing. In Australia
Megan is normally pronounced "Mee-guhn", and so "Tee-guhn" naturally was how that name came to be pronounced in Australia. Because the name was rare outside of Wales, when a character who was an Australian woman appeared on the "Dr. Who" television series in the 1980s, her name was pronounced in the Australian way, and that got people in England and the USA to think that "Tee-guhn" was the correct pronunciation instead of the original Welsh "Teh-guhn". Personally, I wish that had not happened and that the original Welsh pronunciation was what was used everywhere.