It is the Vacuum Reservoir for the HVAC controls. Buy a replacement and get the 2 mounting screws if you don't have those. I don't know what year you have but that looks like the type that screws to the plastic inner fender directly under the air filter box.

In some years, the reservoir fits in between the inner and outer fenders but, IIRC, that type looks a little different from your picture. In either case, it should not be necessary to remove the outer sheet metal fender to replace the reservoir.

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2003 FX4 Level II, Supercharged 5.0L V8, Headers, Duals
BW4406 manual transfer case, Aussie Locker, Torsen L/S, 4.10's
Bilstein 7100 rezzies, Cadillac/Mustang rear discs, Duff traction bars
Keypad Entry, Lock LEDs, ABS switch, sway bar discos
Heated seats, Explorer EATC, Trip Computer and consoles

2003 XLT 2.3L M5OD-R1 reg cab, 2011 Taurus SEL, 2016 C-MAX Energi Plug-in Hybrid