Species: Summoner Cloud
The size will vary depending on the class summoned, but they are a storm cloud ring with portal inside the ring with looks to also be electrified. These rings can be as small as three feet across to over thirty depending on the mana of the summoner and what they are trying to accomplish.
These clouds are not proper Mystic Monsters, while they do have HP they are something of a hybrid between a Mystic Monster and a Battle Spirit; meaning they don't have any direct intelligence and their actions must be directed; although once commanded they will operate on auto-pilot.
All these cloud rings do is navigate a set path, usually a perimeter; set by the summoner. The portal inside it fore spheres that upon contact with the ground expand open rotating arcane sigils of various sizes (based on the size and strength of the Summoner Cloud), that rise up from these sigils random Mystic Monsters of the type "programmed" into the cloud.