In the init.lua file you can enable or disable the spawning of the netherman and dragon by setting nethermobs.NETHERMAN_ENABLED and nethermobs.DRAGON_ENABLED to true or false.
You can also change the spawn heights for the netherman and dragon (for example to allow them to spawn in the overworld and transform the ground to nether sand).
The nether man is the principal habitant of the nether world, it transforms nodes like sand and dirt into nether sand.
The legendary nether dragon is the nether king and breathes deadly fire upon players. It drops a special item with more resistance, called nether dragon scale, and a dragon egg to spawn a tamed nether dragon.
With that item the players can craft resistant dragon scale blocks (including stairs and slabs) and of course the dragon armor (if using the 3d-armor mod).
The tamed nether dragon can only be spawned with a dragon egg. If you right click on it with a saddle, you (as the owner) can start to drive and fly with the dragon. You also can catch the dragon using the lasso.