Strange status bar icon on Samsung devices after using Developer Options
The icon appears to be for when the device is locked to "Portrait" mode. Or in simpler terms, when screen rotation is turned off.
It is most likely inbuilt into the phone and was there before. In case it wasn't, you can disable it from the SystemUI tuner.
Simply tap and hold on the gear (settings) icon which you see when the notifications bar is pulled down. That will enable the SystemUI tuner. In there, select status bar and disable the icon for Auto-rotate screen.