Each of the Prelude and Shadow of Revan flashpoints each have a rare pet or mount that you can obtain if you’re lucky enough!
The 4 Prelude Flashpoints: Korriban Incursion, Assault on Tython, Depths of Manaan and Legacy of the Rakata each have a related rare Mewvorr Pet that can be earned in any difficulty as personal loot from the Last Boss or Bonus Boss in the flashpoint.
Unlike other forms of game content (raids, uprisings etc), you don’t need to worry about playing harder difficulties for a better chance to get these pets, as they seem to have the same drop rate regardless of difficulty. Meaning the easiest way to farm them would be by playing in [Story] difficulty using a Stealth character or by playing in a group.
They can also be traded between players or sold on the GTN.

The Mewvorrs

Thanks to Swtorista for allowing me to source the Pet images from her own website.