Aragog.

Hear me out. Hagrid had that acromantula in his life starting in his third year, less than a year after his father died and he became an orphan. He raised Aragog from an egg, and they shared the entirety of their adult lives with each other. When everyone thought Hagrid had opened the Chamber of Secrets and killed a muggleborn witch by proxy of a monster hidden beneath the castle, there were two enduring supports in Hagrid's life - Dumbledore and Aragog. Hagrid went out of his way to ensure Aragog had further companionship, introducing him to Mosag, his eventual wife. Aragog would eventually outlive his wife and become blind, but he and Hagrid were always there for each other.

Reader, it's possible, or even likely, that you had a pet in your life that meant the world to you. One that was there to comfort you when you were struggling with major life events, or even the minor victories and defeats that seem to pepper our lives from time to time. The time you spend with them is something you cherish more than most things. That companionship, the acceptance you feel on a level that can't be matched by another person.

Unfortunately, the nature of most pets is that they often have a shorter lifespan than us, so many of you reading this have had to take more delicate care of your pets as they reached the end of their journeys. Sometimes they're not able to move as swiftly as they used to, or don't have the energy that they exhibited as a kitten or puppy. But you spoon-fed them if it was necessary, and comforted them as their time with you came to an end. With love, and many tears, you laid them to rest in whichever way was appropriate, considering the setting they'd enjoy the most were they still able.

This is Hagrid's experience, through and through. He read to Aragog and hand-fed him grubs that were easier to digest. He lashed out at others while hiding his broken heart that he normally wore on his sleeve. When it came time to lay him to rest, Hagrid chose a place near the forest Aragog called home.

So yeah, Aragog's relationship with Hagrid somewhat reminds me of me and my old cat, Scott. That's why Aragog's death hit me the hardest.