It could also be a snake from the Forbidden Forest. There are many unknown and mysterious creatures in there. A snake from that forest might be more in tune with the natural magic of Hogwarts. Harry, being the curious and kind - hearted person he is, might come across it during one of his adventures in the forest and start a friendship with it.
It could be Nagini. Even though in the original series she was more of a servant to Voldemort, in the fanfiction, she could have a different personality and form a bond with Harry.
One possible plot could be that Harry discovers the snake has some special magical abilities that are unknown to most wizards. For example, it can lead him to hidden magical artifacts. Another plot might involve the snake helping Harry understand Parseltongue better and use it in unexpected ways to solve mysteries at Hogwarts.
Maybe it's an anonymous gift from someone who has been observing Harry. This person could be a former student of Hogwarts who has a special connection to snakes and sees that Harry has a kind heart and would take good care of the snake. So they leave the snake for Harry to find with a simple note that just says 'For Harry Potter'.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
The snake might be significant in relation to the Slytherin house. Since snakes are associated with Slytherin, it could be a connection to that side of the wizarding world. It could be a symbol of Harry exploring the darker or more complex aspects of magic that are often associated with Slytherin, even though he's a Gryffindor.
A snake could use its venom to attack enemies who try to harm Harry.
It could be a dark wizard who also has the ability to communicate with snakes and wants to use this power for evil. He might be jealous of Harry's connection with the snake and try to disrupt it or steal the snake for his own dark purposes.
The main antagonist could be a Slytherin student who has a vendetta against Harry. When they see Harry with the snake, which they believe should belong to a Slytherin, they start causing trouble for Harry. They might try to frame Harry for using dark magic with the snake or try to steal the snake to prove their superiority over Harry.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
It could still be Harry Potter himself. He has a history of facing dangerous creatures and dark magic, so it would be natural for him to take on the challenge of the deadly snake.