Are daddy longlegs fictional

No, they are real.

"Daddy Longlegs" or "dandy longlegs" are actually called crane flies. They are long, slender, flying insects with long legs.

The "daddy longlegs" or "dandy longlegs" is a nickname for the crane fly. Other nicknames include mosquito hawk, mosquito eater, gallinipper, mayfly, gollywhomper and whapper. The nickname varies depending on country.