Dna Wa Oshiete Kurenai - Animalium No Otome-tachi

From MangaHelpers:
A new kind of living creature has appeared; animals all over the world have suddenly transformed into cute young girls with animal attributes. The main character is a bighorn sheep who transforms into a young girl with sheep horns, who is given the name Yoko. The shepherd who finds her teaches her how to live as a human, and soon he sends her off to find happiness at a school among her own kind.
This school is called "Animalium," and it is full of young girls who were once all kinds of animals, from sheep to wolves to rabbits! Yoko and the other girls will learn and make friends, and they must always try not to get too upset or frightened. If they do, they'll turn back into animal form, returning only to human shape when they calm down! [tethysdust]