The Giant Owl can speak the language Giant Owl:
Languages Giant Owl […]
Notably, this language does not appear on any of the language tables in any sourcebook, but it does appear on the statblock of the Giant Owl and the Skeletal Giant Owl
The second point of the linguist feat says:
You learn three languages of your choice.
Is Giant Owl an eligible language for the feat, or does the feat only allow choosing a language from one of the language tables?