兽 is a bound morpheme (b.f.) meaning 'beast' or 'wild animal'. It rarely stands alone; instead it forms compounds such as 野兽 (yěshòu, wild beast), 兽医 (shòuyī, veterinarian), and 兽性 (shòuxìng, brutish nature). You can use it to describe animal-like or crude behavior in compounds like 兽行 (shòuxíng, brutal act).