I created all the enemy sprites from 3D models myself - used mixamo to animate them and quick-sprites to render them. All the scripts/fsm are my own work. All 3D md3 models were made by me using sketchup.
...but...( for example), the "ed209 droid" is featured in the movie Robocop (even though I made the sprites myself from a model I rigged and animated in mixamo and scripted the FSM) - the weapons are also patented designs by real-life weapons manufacturers (I made the weapon sprites using cc0/public domain models from game-banana) - as are the car brands too (I made the cars and street/building models myself in sketchup using photographs as references). It's taken thousands of hours - all textured, scripted and formatted perfectly for EFPSE.
Are any of the assets direct rips from other peoples games? - absolutely not, however -some of the animals (the tiger and facehugger were not modelled by me - nor the weapons - these are so-called "transformative works" - using premade models (either public domain or purchased from fab marketplace)- which I then converted into sprites, so they can be used in EFPSE).
TLDR-
If you're asking did I create all the sprites/models/sccripts myself - 100% yes - you don;'t need to pay or credit me - are they copyrighted? - no. Are some of the original gun/car models (as mentioned) Trademarked by Real-life manufacturers? Yes - but the work is considered transformative enough to be used in your own EFPSE games (ie a home-made glock pistol sprite does not infringe upon the design-patent of a glock pistol - a low-poly 3D sketchup model of a ferrari does not infringe upon the Ferrari trademark etc)...