A little, simple, colorful jellyfish or fish-like mob that swims in herds in the direction of the planet's ring. In-game, this mob would appear like a small entity (perhaps one block in size) swimming in a specific direction perpendicular to the direction facing the blue gas giant (sorry if that sounds off), as if it is following the flow of the ring.
This guy could bring an interesting ambiance enhancement to the dimension that is connected/related to the ring planet part of the mod.

• What if it collides?

The mob could have an AI tweak where whenever it collides with an obstacle, it would swim in a direction perpendicular to that obstacle until there isn't anything blocking its way to the flow and just go on. Think of it as if it is swimming around the obstacle.

• Extra:

To give this mob purpose (and doom), a cruel player could kill it to obtain a part of its body as an item. For example, if it is a jellyfish, it could be a mesoglea tissue or its tentacles. This item can be brewed into a potion which effect will make players float in a direction mimicking that of the mob. The effect could trigger whenever a player falls off an island saving them from the void demise. It could also trigger when the player jumps from a high place allowing them to use the effect as a form of traveling between islands. To avoid getting dragged too far, the player can sneak to slowly descend.

NOTE: The potion is meant to be used inside the dimension, so the effect won't work outside of the dimension since it makes sense (it is related to the mob's dependence on the ring's gas), and it is meant to help with traveling in the dimension. If a function outside of the dimension is desired, then if the player triggers the effect in another dimension, instead floating in a specific direction, they will flow in a random one (not down nor directly up, mostly horizontal and close to the direction the player is facing).

One more thing, for entity-to-entity interaction, the mob would respond in either an aggressive or docile way:
Aggressive - when colliding with another mob (not the same kind) or a player (you), the mob would either give a damaging electric zap or a poison effect. The interaction shouldn't necessarily be deadly, and just to let you know that you are getting in its way. You can think of it as a man o' war jellyfish sting but less painful.
Docile - upon collision, the mob could just give a little push and move around.

Here is an ugly concept sketch as a reference (yes it is from MS Paint, sorry)