While traveling to Nevarro, Grogu sees the silhouettes of a group of creatures in hyperspace and these creatures are quite significant in Star Wars lore.
Under normal circumstances, when a ship travels into hyperspace, it is to make a quick escape or travel vast distances. Other ships cannot pursue a ship in hyperspace unless they have very special capabilities that have been established in the universe.
The creatures that Grogu sees while Din Djarin is asleep are closely tied to hyperspace travel and force-sensitive individuals in particular, and they are called the Purrgil.
The early pioneers
Purrgils have been mentioned before but their first true appearance was in Star Wars: Rebels. They are semi-sentient species that closely resemble whales and have a trail of tentacles on their bodies.
The regular purrgils are the size of a small starship but there is a sub-species called Purrgil Ultra that is much larger. Purrgils travel in groups across star systems and have the natural ability for hyperspace travel.
This natural ability is what inspired early sentients to create hyperdrive technology and recreate hyperspace travel.
One of the reasons Grogu may have seen them specifically is the fact the force-sensitive beings can form bonds with other creatures and another Jedi, Ezra Bridger, formed a very close bond with purrgils and defeated Grand Admiral Thrawn with their help during the attack on Lothal.
These creatures were considered nuisances for regular pilots as they would regularly fly into the flight paths of ships, causing them to crash to their deaths.
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