Sprig of wolfsbane
Wolfsbane is a vegan food, and is also suitable food for herbivorous pets.
Priests begin the game with 1 or 2 sprigs of wolfsbane.
This is in fact a major case of NetHack is not real life, because wolfsbane in real life is very poisonous (so don't eat it in real life), and the name comes from the fact that it was used to poison wolves. Wolves were often killed by Christians for two reasons: first, wolves sometimes prey on livestock and were considered a pest, and second, wolves were considered sacred by Germanics (for example Wotan is accompanied by the wolves Geri and Freki), and the monotheist Christians desecrated their values, by killing wolves (or chopping down old oaks which were used to worship Tyr).