Telestial is a neologism (a “new word”) that was coined as part of the revelation of D&C 76.
Celestial and terrestrial are both derived from Latin. The word caelum means “sky, heaven”; turned into an adjective, that form becomes caelestis “heavenly”; and apparently that ending was extended with another Latin adjectival ending, -(i)alis, to get the form caelestialus. The Latin ending -us was dropped when the word was anglicized (and the ae diphthong was reduced), giving us “celestial.”
Terrestrial underwent a similar evolution. Terra means “earth”; terrestris is the adjectival form, “of or relating to the earth”; and terrestrialus would be an extended adjectival form, with the -us ending being dropped in English, for terrestrial.