That's the title of this short video (1:13) in which Professor Jeff Karlin of Golden Gate University explains that the IRS distinguishes between killer whales and great white sharks, although both are big giant swimming things that could potentially eat you:
Sadly, the video doesn't go into much detail, which is unusual because it appears to be designed to encourage people to go to law school, which as some of you know involves relatively fun and interesting topics like this one absolutely every single day.
My guess is that he was discussing classification of gains and losses from the sale of business property under section 1231 of the tax code. There is a special rule for "livestock" (1231(b)(3)), which says cattle and horses are business property if you have had them for particular purposes for more than 24 months, but "other livestock" only have to be held for 12 months. It also says poultry aren't "livestock," but leaves the question of how to define "other livestock" a perplexing and frustrating mystery.
Luckily, Treasury Regulation §1.1231-2(a)(3) comes to the rescue:
For the purposes of section 1231, the term "livestock" is given a broad, rather than a narrow, interpretation and includes cattle, hogs, horses, mules, donkeys, sheep, goats, fur-bearing animals, and other mammals. However, it does not include poultry, chickens, turkeys, pigeons, geese, other birds, fish, frogs, reptiles, etc.
I would assume that poultry et al. are thus subject to the general rule, not this special one for "livestock." The question of why the lines are drawn this way is surely a totally fascinating one but involves way more research than I care to do on a Sunday afternoon. (To be honest, so did this.)
In any event, as Prof. Karlin explains, an orca is a mammal and so is classified as "livestock," while the dead-eyed remorseless killing machine roaming the seas in search of human prey that is the great white shark (source: "Jaws 3-D") is a fish, and so is outside the livestock category along with frogs and chickens and so forth.
As to how this may affect your plans to start your own great-white-shark ranch, I can't advise you on that. Please consult a tax expert and a psychologist.
Via Martin Gardiner at Improbable Research.