I was wondering about the earliest use of telepathic bonding in both fantasy as well as science fiction. I guess I should narrow it down by excluding simple telepathic 'speaking'. So, I define it as two characters sharing both mental 'voice' and mental 'emotion' telepathically in the context of a friendship. I'd separate out human-human, human-animal(otherwise 'normal'), human-animal(having essentially human level intelligence), and human-alien intelligence. Someone mentioned E. Hamilton's 1948 story The Valley of Creation, but I am unfamiliar with that - although I do know it involves intelligent telepathic animals. A. Norton's 1952 StarMan's Son fits the 'normal animal' bonding, iirc. Anything earlier? And how about the other 3 types? In sci-fi? In fantasy?