There are many living things that only have four letters, off the top of my head in one minute I came up with the following (only one is plural since you said "a living thing")...
ants, aunt, bird, bush, calf, crow, dame, fish, frog, goat, tree
sear
gold dust
Envy Is A 4-Letter Word
Salt is a 4 letter word for seasoning.
4 letter word for drum is "Dhol".
cell
a toad
sear
lion, lamp, list
gold dust
sell, sway
seed
Envy Is A 4-Letter Word
4 letter thing that ends with m is SPAM.
Salt is a 4 letter word for seasoning.
4 letter word for drum is "Dhol".
4 letter word for final