Q- Is there a narration stating that the dog mentioned in the incident of the people of the cave will go to Jannah?
Have not come across any Hadith of Rasulullah (sallallahu ‘alayhi wa sallam) which mentions this. Certain Tafsir books and non-primary Hadith sources have cited this as the statement of the Tabi’i; Khalid ibn Ma’dan (rahimahullah). (Al Lubab Fi ‘Ulumil Kitab, Surah Kahf, Verse: 18, Tafsir Abu Sa’ud, Surah Kahf, Verse: 18, vol. 5 pg. 238, Ruhul Ma’ani, Surah Kahf, Verse: 18, vol. 15 pg. 237, Hayatul Hayawan, vol. 4 pg. 345)
However, I could not locate a chain of narrators for this statement of Khalid ibn Ma’dan (rahimahullah).
And Allah Ta’ala Knows Best
https://hadithanswers.com/will-the-dog-of-ashabul-kahf-be-in-jannah/