What does ほんとにあった mean? (translation), the context appears in the image but I explain: they went to the shore of a lake to look for herbs for their friend who is sick, and the bird, thanks to being sensitive to water, found the shore and took them and when they arrived say that
This ほんとにあった means "(I realized) it really exists!", "It's really here!". This person had been skeptical, but just realized the bird was correct.
This あった is something called modal-ta. It's used when the speaker found something. See: Non conventional usage of the past tense
