Support is exclusive to MythicMobs 5 and above. If you have an older version this wiki may be inaccurate.
You can spawn mythic mobs in custom biomes by following this format. View a larger explanation via Entities.
{
"type": "CAT",
"surface": "LAND",
"specialType": "MythicMobs:JumpingSpider"
}
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 names installed on the server if you are using Iris Studio, too!
See the MythicMobs wiki:
More specifically: