Thestrals could also be very useful. Since they are invisible to most people but not to Harry (if he can talk to them), he could use them for stealthy travel. They are also very strong and fast, so they could carry Harry and his friends to places quickly and without being detected.
Hedwig would be a very useful ally. Since she can fly and is already Harry's pet, she could be his eyes and ears in the sky. She could deliver messages faster and also gather information from other animals she meets during her flights.
Hippogriffs are popular as well. Their majestic appearance and the fact that they require respect to interact with make for interesting stories. Fanfictions might explore a young wizard's journey to tame a hippogriff and the bond that forms between them. There could be a story where a hippogriff is wrongly accused of attacking a wizard, and the main character has to prove its innocence.
Hedwig is likely to be one of the main animal characters. She was Harry's owl and had a big role in the original series, so in the fanfiction she might have even more expanded storylines. Buckbeak the hippogriff could also be a main character. He was already quite important in the books and could have new adventures in the fanfiction.
Dumbledore could be a very likely ally for Loki. Dumbledore is known for his wisdom and ability to see the good in people. He would recognize Loki's power and his potential for both good and bad. Dumbledore could guide Loki in understanding the values of the wizarding world and in return, Loki could offer his unique skills and knowledge. Dumbledore would also be able to keep Loki in check if he were to get too carried away with his mischief, and together they could face threats to the wizarding world that require a combination of Asgardian and wizard magic.
The owl is also popular. Owls are already an important part of the wizarding world as messengers. If Harry turned into an owl, he could travel quickly between places and deliver important messages himself. It gives a new aspect to his role in the story.
His closest ally could be Professor X. Since Professor X is also very much about using his mental powers for good and has a great understanding of powers and how to control them. He would likely see the potential in Harry and help him develop his future - seeing ability further while Harry could use his ability to help Professor X in his plans for mutant - human coexistence.
In the fanfiction, Ally Martin could be an original character created by the author. She might be a witch or wizard with her own set of abilities and personality traits.
Well, it could change the plot in many ways. For example, he might get early warnings about danger from animals. If a snake is about to attack someone in the Forbidden Forest, the snake could tell Harry, and he could prevent it. This would make him seem even more of a hero and might change the way his friends and enemies view him.
Asriel would be a great ally in terms of raw power. His abilities are on a different level compared to Hermione's. He could provide a lot of brute force when it comes to fighting against dark wizards. However, Hermione is extremely intelligent and well - versed in wizarding lore, which is also very valuable.
It could be a fanfiction where Harry is given a special charm or potion that enables him to talk to animals, and he embarks on a quest with their help to uncover hidden secrets in Hogwarts.
In some fanfictions, there might be new characters introduced who are also involved in the world of animal communication. Maybe a young witch or wizard from a different part of the wizarding world who has a special connection to a particular type of animal and teams up with Harry to explore the full potential of his ability to talk to animals.