A Brother to Basilisks by Lomonaaeren

A Brother to Basilisks by Lomonaaeren AU of PoA. Harry wakes in the night to a voice calling him from somewhere in the castle—and when he follows it, everything changes. Author's note: This is a canon-divergent AU that starts after Chapter 7 of Prisoner of Azkaban. The title is based on a quote from the Book of Job: “I am a brother to dragons, and a companion to owls. This is a long story and, at the time of writing this rec, is a WIP, however the author always finishes her works so it will be completed. This is one of her complex and fascinating stories that explores a HP universe reacting to Harry Potter with a basilisk as a bonded companion. So lots of political shenanigans and side taking and some wonderful snarky asides from Dash. Ron and Herimione are constant and true friends, while other character's attitudes to Harry are changed by his bond with Dash, which makes for interesting and sometimes surprising reading.

Lomonaaeren has written nearly 500 stories for Harry Potter. Many are focused on  H/D as adults, but she has written other pairings and a number of gen stories, which are usually excellent. With that many stories it is not surprising if sometimes you feel you are reading the same story slightly altered, however the vast majority of her stories are totally original and they are always well written.