Is it possible to have 3rd party monsters?

[Player T]

So, I'm currently working on a Grey Alien, and I would like to know if it is possible to make the monster hostile to demons and players alike. Here is the unfinished version I used for this screenshot Xinian Imp Replacement.zip

 

[SMG_Man]

Using a more advanced map format like Doom in Hexen or UDMF (rather than vanilla or boom format), you can implement this through use of the Thing_Hate special.

[Player T]

I know, I should've mentioned that in the post, however I'm talking about making a 3rd party monster that doesn't use the Thing_Hate special. 

[SMG_Man]

ah. From what I've seen then, I don't know that it's otherwise possible through DECORATE or ACS alone. It is possible that you could make custom behavior through ZSCRIPT, but that's outside of my realm of expertise.

[Player T]

Well, damn. Oh well, thanks for the help!