If it's a real animal at all, it is most likely an elephant, the word described as a "tail" in Hebrew doesn't necessarily mean "tail". In its basic sense the word simply means "extremity", as such it might be describing the trunk of an elephant. One interpretation, in fact, is that this is a reference to the animal's...well...penis.
It's also possible that the behemoth and leviathan are both legendary beasts. Jewish folklore recognizes three great legendary creatures, the behemoth, the leviathan, and the ziz. The ziz is not mentioned in Job, but is possibly mentioned here in Psalm 50:11,
"I know all the fowls of the mountains: and the [ziz] of the field is mine."
The word is sometimes translated as "wild beasts" or "all that moves"; but is possibly a reference to the mythical ziz (the word is itself right there in the text after all), and is mentioned alongside birds.
In this case behemoth isn't a reference to a real animal, but to a legendary/mythological creature which was readily familiar. It's the same way that the use of "dragon" gets used in the Bible, not to literal dragons which do not and never have existed, but which were well known mythological creatures and thus could be utilized for literary purposes. The behemoth and the leviathan, therefore, are used in Job as grand expressions of God's creative power; it places God above even the mightiest and most fearsome creatures of human imagination. After all, God declares, "Where were you when I created the heavens and the earth?" Thus the point in Job, as wisdom literature, is to extol the proper fear of God, the almightiness of God in all things. Job, after all, isn't history, but wisdom--like Proverbs and Ecclesiastes.
-CryptoLutheran