How to mount a derailleur hanger on this frame? I have acquired an older Pegasus, model unknown. Guessing by the decaying decals it might be something like Atlas. Bike is in poor neglected condition, but I will try to put in on the road to see how it rides and then decide if I dump it or do some more work on it. It came with a Suntour AR II rear derailleur. The frame has simple dropout and bike came with some sort of generic deraillur hanger. The problem is that when I release the quick release the hanger with derailleur comes loose along with the wheel. Below are some pictures. As you can see the derailleur has no clear way of mounting to the frame. The frame does not have any hole that aligns with small the in the hanger. The hanger has on the backside a little edge that aligns it with a dropout as seen on the picture. 
Options I considered were:

1. Drilling a hole in the frame and then cut a thread for the bolt. Which thread should I cut in that case to be "universal" or compatible with bolts that are used for that purpose?

2. Puting a bolt through the small hole in the hanger and through the dropout. Use a big washer and a nut on the backside to fix the hanger in a dropout semi permanently. I would then fix wheel axle with quick release as usual. Down side of it is, that the axle would be moved forward in dropout by about 14mm.

3. Try to find a suitable hanger. No luck so far. Through Google I have found a picture of a hanger that might do the job.

What would my best or simplest option to mount that hanger?
You have answered your own question, correctly too. 

Those generic hangers are exactly what you want.  The golden piece held on with the short bolt should slide up the dropout and rest against the curved end, and your wheel axle clamps down on the hanger.

Then your rear derailleur will thread into the large 10mm hole in the hanger.

Its not complex, but it can be fiddly with tolerances.  Good work, all I can do is confirm you're on the right track.