At the core of Japanese there’s hiragana, the basic Japanese alphabet (or syllabary, if you want to be pedantic about it). But it wasn’t until pretty recently that hiragana was standardized. Until the 20th century, people could basically write hiragana however they wanted to. Those different ways of writing hiragana were called hentaigana (変体仮名).
Source: “Tofugu” | Subject: History