The fictional language J.R.R. Tolkien created with strong influence from Finnish is Quenya.
Quenya is one of the major Elvish languages in Tolkien’s legendarium. Tolkien admired the sound and structure of Finnish, and its influence can be seen in the development of Quenya, although the language is not simply Finnish with invented vocabulary. He also drew on Latin, Greek, and other linguistic sources.
Within the stories, Quenya is associated especially with the High Elves, including the Noldor. It functions partly as an ancient or ceremonial language by the time of The Lord of the Rings. The Elvish language more commonly spoken in much of Middle-earth during the Third Age is Sindarin.
Tolkien was a professional philologist, and his languages were designed with historical relationships, sound changes, and grammar. This linguistic depth helped make Middle-earth feel like a world with its own long cultural history.