You can use Iris to spawn MythicMobs by using the following format in your Iris Entities.
The "type": "CAT"does not actually change anything. It just makes sure Iris accepts the entity.
As seen here, to make Iris spawn a mythic mob, you'll need to create a new Iris entity file with specialType, where you prefix the name with MythicMobs:
VSCode will autocomplete Mythic Mob names installed on the server if you are using Iris Studio, too!
Due to MythicMobs not supporting custom biomes, it can be hard to use MythicMobs' random spawns to spawn mobs within Iris biomes.
However, you can use this small side plugin to add support for Iris biomes called RealBiomeCondition. This was made by a developer that contributes to Iris for the exact reason of supporting Iris biomes.