The term "long horse" is not commonly used in modern English. However, it may refer to a few things, such as a large horse, a piece of wood used for punishment, or a nickname for snakes. Synonyms for a large horse could include stallion, mare, gelding, or steed. If referring to the punishment tool, the synonyms might include a cane, switch, rod, or birch. As for snakes, some alternatives could be serpent, reptile, viper, or cobra. It is essential to note that using synonyms always depends on the context and the intention behind the usage of the word.