I have to disagree with the comment about -tomy: the range of meanings of the Greek tem-/tom-/tm- root is quite wide and includes the required "cutting into". Many medical terms include -ec-tomy with a separate preposition "out", but plain -tomy as in 'neurotomy' can mean "cutting (through)".
The word would seem to have been coined by Morris Marples in his book White Horses and other Hill Figures of 1949. As many subscribers have pointed out since this article first appeared, Morris Marples was no Greek scholar. It looks as though it is formed from the Greek roots leuci–, white, hippo, horse, and the suffix –tomy. Unfortunately this last doesn’t mean cutting or carving, but refers to cutting out or excising (as in many medical terms such as hysterectomy), so it actually means cutting off or excising white horses, which isn’t the same thing at all. (And in any case, it’s short one p and has one too many ts.)