From The TimeSplitters Wiki

Aliens are beings from another planet.

Here are some:

Meezor Mox

Ozor Mox

koozer Mox

Pillar Alien

Float Alien

Red Alien

Female Alien

All of these aliens appeared on Planet X at some point, but the Mox family inhabit it in TS2, whilst the other assorted aliens are in TS1's Planet X.