I want to create an ontology for my game application but it seems to me that I always have to choose an IRI for my ontology and then share the ontology online.
How can I create a local ontology with local subject IDs that have relations inside the application ?
Choosing an IRI for your ontology does not mean the IRI has to be a URL or that the URL needs to be resolvable and provide the ontology on retrieval. A URN will do - e.g., urn:games:myGame is valid ontology IRI.
Once you save your ontology to a file, you can load it through the file name. This removes all needs for the ontology being shared online.