> I know how to pronounce 'Kansas', and I was always under the impression that 'Arkansas' is the same with an 'Ar-' prefixed, which is apparently not the case.
The reason why they're pronounced differently is that they have different language origins. Arkansas is from the plural form of the French name of a Native American tribe, so you don't pronounce the '-s' while Kansas comes from the English spelling of a similar(?) tribe, where the '-s' is pronounced. English is notorious for just borrowing words from other languages wholesale but it seems to work most of the time.
The reason why they're pronounced differently is that they have different language origins. Arkansas is from the plural form of the French name of a Native American tribe, so you don't pronounce the '-s' while Kansas comes from the English spelling of a similar(?) tribe, where the '-s' is pronounced. English is notorious for just borrowing words from other languages wholesale but it seems to work most of the time.