It could be Voldemort. Imagine he gets wind of Harry having this powerful Wolf Zord. He would stop at nothing to get it for himself, seeing it as a means to finally defeat Harry and take over the wizarding world completely.
I haven't read this exact fanfiction, but generally, it could be that the author wanted to create a crossover between two very different universes. Harry Potter finding the Wolf Zord orb could lead to a story where he has to deal with new powers that he doesn't understand at first. He might face challenges from both the magical world and whatever threats come with the orb. For example, there could be villains from the Power Rangers side who are after the orb and Harry has to protect it while also trying to figure out what it can do.
Perhaps Harry is transported to a different dimension where the Power Rangers exist. There, he discovers the Wolf Zord. He has to learn how to bond with it, just like the Rangers do. He could bring some of his magic skills to the table, and in turn, the Wolf Zord gives him new abilities to use against Voldemort or other foes back in his own world.
There are also some specialized Harry Potter or Power Rangers fanfiction communities. Check out forums where fans discuss and share their works. They might have a link to this particular fanfiction. Another option is to do a general Google search with the title in quotes. Sometimes Google can lead you to some lesser - known fanfiction repositories where this story might be hosted.
It could be a dark wizard who wants to get his hands on the lightsaber for its power. He might see it as a new tool to use in his evil plans, like using it to break into Gringotts or to fight off any who oppose him.
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 be a dark wizard who also wants to get to the secrets in the lost library. This wizard might try to stop Harry from exploring the library and getting its knowledge.
It could be a new dark wizard who has been secretly searching for the Death Stone. Maybe someone who was in the shadows during Voldemort's time and now sees an opportunity to rise with the power of this stone.
Perhaps there's a group of hunters in the fanfic. They don't care about the wolf pup's connection with Naruto. All they see is a valuable catch. They might try to set traps for the pup and Naruto would have to outwit them to keep the pup safe. They could be motivated by money or a desire to prove themselves as the best hunters in the area.
In this fanfiction, the main antagonist could be an ancient evil that has been trapped within the grimoire. Once Harry finds the grimoire, this evil starts to gain power and tries to use Harry to break free completely and spread chaos in the wizarding world.
There could be a group of magical creatures who see the snake as an abomination or a threat to their kind. They would try to destroy the snake and Harry would have to protect it. These creatures could be from a part of the Forbidden Forest that has not been explored much in the original books and they have their own set of rules and magic which makes them a formidable antagonist.