Some space monsters

Angels of Iego
Type: space encounter
Description: 'Star Siren'
Native to Iego and it's moon, these beautiful humanoids were known to entice star travellers to crash on their planet, to remain with them forever.
Source: Phantom Menace, Clone Wars

Duinuogwuin
Type: space encounter
Description: 'Star Dragon'
Powerful spacefaring dragon-like race. Blockaded the Republic star lanes early in the Republic's history until a Jedi-sponsored truce. Some are known to be Force-sensitive.
Stages (Optional): Very powerful. No lair or treasure as their home is unknown. Might grant knowledge of a distant system or a secondary quest.
source: Legends

Exogorth
Type: space encounter
Description: 'Giant Asteroid-dwelling Space Tube Worm'
Eats star ships and pretty much anything. Fairly immobile and slow to attack.
Stages: Dwelling asteroid will contain +3 bio if the beast can be cleared or tamed. Might contain a star ship and/or Mynocks if defeated/put to sleep and robbed.
Source: The Empire Strikes Back

Mynocks
Type: space encounter
Description: 'Space Barnacle'
Space-faring beasts which attach themselves to spacecraft and drain their power.
Source: The Empire Strikes Back

Neebray
Type: space encounter
Description: 'Space Manta'
Space-faring herd beasts which while generally non-agressive, can be a hazard to navigation
Source: Clone Wars

Purgill
Type: space encounter
Description: 'Space Whale'
Starfaring herd creatures with built-in hyperdrive. They need to ingest a apecific gas, Clouzon-36, to power their innate hyperdrives. They will be found congregating around energy-rich asteroid fields if this gas is present for them.
Stages: No treasure or lair known. Hazard to space navigation. May aid space travellers in distress.
source: Rebels