A Well Cap is sometimes referred to as a "turtle" because of its unique shape.
This style of well cover is the most common.
It has a unique bump-out which covers the electrical conduit (usually plastic) with wires for the submersible pump.
Turtles may be made from plastic, zinc, aluminum and cast iron.
Some have side-bolts which clamp the turtle to the well casing, and others screw from the top, into a ring with threaded holes.