Diode in MOSFET symbol
Actually all MOSFETs have two (parasitic) diodes.

This shows how a MOSFET on a chip can be constructed:



Note that both the Drain and the Source have a PN-junction to the Substrate (the purple region). These PN junctions form a diode, so between Drain and Substrate there's a diode and between Source and Substrate there's a diode.

HEXFETs and other Vertically oriented MOSFET devices might have a different buildup but the PN junctions, the Drain-Bulk and Source-Bulk diodes will always be present.

Most discrete MOSFETs do bot have a separate Substrate connection, instead the Source and Substrate are shorted. To use the MOSFET properly this needs to be done anyway so it is done in the package.

That leaves the Drain-Substrate diode and since the Source and Substrate are shorted, this diode sits between the Drain and Source contacts.

As you have noticed, some MOSFET symbols include this Drain-Source diode in the symbol just for clarity, others don't. But no matter what the symbol shows, that diode will always be present as it is an intrinsic structure of any MOS transistor.