animalo

A loan word sitting next to a native one, and the difference is register.Besto at rank 373 is the everyday word: a creature that is not a person. Animalo is the scientific one: the kingdom Animalia, the whole branch of life that feeds on organic matter and can usually sense and move. In that classification human beings are animaloj and the bestoj are the subdivision that excludes us. Which is exactly the English split between "animal" in a biology lecture and "animal" in a pet shop, except that Esperanto gives each sense its own word.

So the working rule is short. If you are talking about a dog, a horse or the things in a zoo, say besto. If you are writing biology, taxonomy or a sentence where humans have to be inside the category, say animalo. Bestoj kaj homoj is a normal pairing in ordinary Esperanto; animaloj kaj homoj reads oddly to a scientist, because the second group is inside the first.

One honest warning about usage. Reta Vortaro records a second sense in which animalo is simply a synonym of besto, and marks that sense evitinde, to be avoided. That is the dictionary telling you what the loan word tempts speakers of English, French and Spanish into doing: reaching for the word that looks like home and flattening the distinction the language built. Resist it. Using animalo for the neighbour's cat is not an error anybody will fail to understand, but it wastes a word Esperanto took the trouble to keep separate.