The VGA connector on the Naomi is not entirely standard. Unlike most older game hardware it does maintain the VGA-standard signal levels, so it can be used on any VGA monitor, but it puts out +5V on pin 9.

Pin 9 has been used for several things in the VGA connector's history. Originally this was the KEY pin, which helped orient the connector. In 1987, IBM started pushing 12 volts through it. In 1996 VESA redefined pin 9 as +5V.

If your VGA cable heats up when used on a Naomi, it probably has pin 9 wired to ground, either in the cable or at the receiving end.

Some of the unlabeled pins may have additional functions (such as monitor ID, etc) but for Naomi use, they're not important. If you need to know the function of those pins, you can find them on a standard VGA pinout.