This year's Halloween edition of Fantastic Companions contains five different monsters in five different levels each (for a total of 25 statblocks). Each of these is inspired by a vampire family from Bestiarium: Scales & Decay.
They can all be companions to the player characters and evolve along with them. The document also includes the general rules for Ready to Play Fantastic Companions, as optional rules for your campaign.
The goal of these rules is not to make the combat system more complex, but rather to facilitate the rules to emulate the fantasy of having fantastic companions in a 5th edition adventure.