Ido
Ido is a language that came from Esperanto, so they look very similar. Since Esperanto has more speakers than Ido, most people that know Ido first learned Esperanto and then later learned that Ido is a language, too. Sometimes Idists (people who speak Ido) and Esperantists (people who speak Esperanto) do not agree with each other. Luckily they both agree that making a language that everybody can learn is a good idea. Most Idists and Esperantists can understand most of each other's language.