Is Umayuru: Pretty Gray hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

ONA · 2025 · 4 episodes · PG-13 - Teens 13 or older
Anthropomorphic, Comedy
A short animation spin-off series that focuses on various ashen-haired Umamusume. (Source: Uma Musume Pretty Wiki)
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