How do I remove this back wheel axle nut? 90s Peugeot mountain bike It's a 16mm nut that won't turn, and I don't want to bend my cone wrench. 

The other side came off easily but was a normal nut. This side seems to have some sort of lock/ hole for a pin but I'm not entirely sure what to search for in regards to it 


http://s29.postimg.org/3v75cum3r/image.jpg

I've uploaded a picture, notice the little dip / semi circle hole on the end of the axle? I'm pretty sure this is related to whatever's stopping me from turning the nut
That lock groove is for a washer between the cone and the locknut, and there's no way to have a nut on the axle that locks into it. So you can ignore it.

What you need is a proper 16mm ring spanner. Even an adjustable wrench could well strip that nut. If you have to go to a bike shop or some other workshop to use one, do that. Or just buy one, since you're likely to need it.

Commonly those nuts are done up ridiculously tight for a reason. Often it's because that derailleur protector is only held on by the one nut, so if bumped it will tend to rotate and loosen the wheel nut (the one you're trying to undo). Rather than getting a proper derailleur protector (and that keys onto the frame), a "solution" is just to do that nut up really tight. Which also puts pressure on the cones, so people will often back the cones off a little to stop the wheel binding. It's a classic case of "now you have two problems" (or here).