Kosumo- is clear enough - it is cosmo-. But zo:n is rendered into English everywhere, including the Osamu Tezuka World website and official publications, as "zone", which makes no sense, because it is absolutely clear from the dialogue that kosumozo:n is a creature, not a place - one cannot fight and capture a zone.
Meaningless pseudo-English is common enough in Japanese mass culture, but it turns out that the kosumozo:n is different. An early tie-in book, The Big Phoenix 2772 Encyclopaedia, mentions in passing that Osamu Tezuka himself explained the meaning of this word as "space lifeform" (Hi-no Tori 2772 Ai-no Kosumozo:n Daihyakka. Tokyo: Keibunsha, 1980, p.223).
So this is the answer: zo:n is not English, it is zoon, the Greek for "life", as in zoology or protozoa. Tezuka with his medical background and fondness for classics coined an apt term to describe the Phoenix - something that lives in space, as well as the Universal Life.
It is a pity that he did not correct the mistranslation.