DRD Team

That is possible?
I mean, how to Spawn a monster as Deaf Thing?

If this isn't possible, go to Feature Suggestions.

It's actually an ambush flag, (calling it the "deaf" flag wouldn't be totally right) and yes it is possible, if the parent object has one itself. There's a flag for transferring the ambush flag, which can be set on the fly before it even spawns.

More info here:
http://zdoom.org/wiki/A_SpawnItemEx (SXF_TRANSFERAMBUSHFLAG)
http://zdoom.org/wiki/A_ChangeFlag

Of course, these are decorate functions, making it work with Thing_Spawn might be a little more complicated, but I assure you, is quite doable. Just I can't think of a very clean way to do it.