The plot might be that Pitch has a new plan to create chaos. He discovers that turning Jack into a baby pooka can disrupt the order of the magical world. Once Jack is transformed, he is found by a group of friendly forest spirits. These spirits don't know who Jack really is at first but take him in. As time passes, Jack starts to show some of his old abilities in small ways, like being able to create a tiny bit of frost. This catches the attention of the Guardians who then start to investigate. They eventually figure out it's Jack and have to fight Pitch to get the antidote to turn Jack back.
Jack's personality would likely become more innocent and naive. As a baby pooka, he might be more curious about the world around him without the usual wariness he has as his normal self. He could be more easily frightened by things that he wouldn't normally be afraid of as an older Jack. Also, he might be more impulsive in his actions, just going for things that catch his eye without thinking much about the consequences.
Perhaps in this form, Jack Frost is befriended by a young human child. The child doesn't realize he's Jack Frost and just thinks he's a really cool and strange little animal. They go on adventures together in the winter wonderland, with Jack Frost using his powers in small, unnoticeable ways at first.
One interesting story could be about Jack Frost teaching Baby Pooka how to create unique ice sculptures. In this fanfiction, Jack Frost uses his powers over ice and snow to show Baby Pooka the beauty of his creations. Another could be a story where they team up to save their magical realm from an encroaching darkness. They use their individual abilities, Jack's cold powers and Baby Pooka's special skills, to fight off the evil forces.
The plot could center around Jack Frost being cursed into his baby pooka form by a mischievous winter spirit. He then has to find a way to break the curse. Along the way, he meets other magical beings who either help or hinder him. For example, a snowman he created in his normal form might come to life and guide him to a magical crystal that has the power to reverse the curse.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a wide variety of fan - created content, and you can search specifically for Jack Frost and Baby Pooka stories using the search function. Another option is FanFiction.net, which also has a large collection of fanfiction, though the tagging system might be a bit different.
One common theme is friendship. Since they are two different characters from similar mythological - inspired backgrounds, their journey to become friends is often explored. For example, they might start off as strangers but gradually learn to understand and support each other.
Another theme is discovery. Usually, either Dark Pooka or Little Jack discovers something new about themselves or the world around them. In some fanfictions, it could be a new magical power or a hidden place. This adds an element of mystery and adventure to the stories.
One possible plotline could be her journey in a magical world filled with other mythical creatures. As a female Jack Frost who is also a Pooka, she might struggle to fit in or find her place among the different magical beings.