That's because some are Indigenous names phoneticized for English speakers (Yonge and Markham on the other hand are entirely English names):

Etobicoke. From Adobigok [1]

Tecumseh (or Tecumseth). From tecumtha or takhamehse [2]

Mississauga. From Misi-zaagiing [3]

[1] https://www.etobicokehistorical.com/brief-history-of-etobico...

[2] https://en.wikipedia.org/wiki/Tecumseh

[3] https://en.wikipedia.org/wiki/Mississauga