In your Room of Requirement, you can rescue and pet a number of mythical creatures. Beasts may be trained, tamed, and even bred. To breed your beasts, complete the Plight of the House Elf quest and the Rookwood Trial, then talk with Deek.
Thestral will be given to you in the Foal of the Dead quest, which requires you to purchase a Breeding Pen from the Spells and Tomes shop, and capture a male and female Thestral. Once you have purchased the Breeding Pen from the Spells and Tomes shop, go to your Vivarium in the Room of Requirement.
How to Finish the Hogwarts Legacy Plight of the House-Elf Quest
How to Include Breeding Pens in Hogwarts Legacy
Once you have reached your Vivarium, equip and use the Conjuring Spell. Select the Beast Items tab and then select the Breeding Pen. It will require 18 Moonstone to place. If you have a male and female beast in that Vivarium, activate the Breeding Pen. The process takes 30 minutes to produce a beast offspring.
In Hogwarts Legacy, how do you sell beasts?
Sometimes you may want to sell some of your animals for extra gold. If your Vivarium is becoming too crowded, you may want to sell a few. Visit the Brood and Peck shop in Northern Hogsmeade and ask for the inventory icon.
All Hogwarts Secrets and Where to Find These in Hogwarts Legacy here on Pro Game Guides.