An enumerated field will be represented most likely as an integer value in the row, so keep that in mind. Otherwise, you are going to use a foreign key lookup

An enumerated field will be represented most likely as an integer value in the row, so keep that in mind. Otherwise, you are going to use a foreign key lookup to get your type, which will also be an integer stored in that same row. But, if you wanted to add a new animal type, and you used a lookup table, even if you have billion of rows, you are not messing with the table with a bunch of data. That may not be the case with an enum. It sucks to ALTER a live table in production with billions of rows being actively INSERTed.