It is a reflection on the misery of a tree.
The quote is a riddle known to many in English-speaking countries. The phrase has a hidden, elusive meaning due to its intentionally incorrect grammar. It is apparent that some of the words in the sentence are missing or omitted. Perhaps, it is done so that the reader could work his or her imagination trying to guess what this saying could mean.
The most feasible explanation is that “If trees had horses would be single” is a philosophical piece on the stationary nature of trees. In literature, inanimate objects are often given human characteristics. This could be the case. Maybe a tree in a forest envies those creatures who can move freely and wishes it could at least have a means of transportation of its own and be a traveller.