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While JavadocMD's comment/answer may work. (Assuming you can have them kill it, if not you could still throw it in a deep pit.) This would usually result in mutilating the corpse, or maybe your dwarves would bury it.

The easiest and probably best way would be to train it as a war/hunting animal at the kennel. Then it will show up in the animal list and can be butchered.

As for why it won't normally show up, it's probably a bug related to the fact that you bought it.

The date is not currently something that can be displayed. Though I do not know of any currently, I think it should be possible to display the date via a mod -- perhaps you should try asking on the Modding section of the bay12 forums?

Do note, that the date is also mentioned for any given Announcement, if you're trying to determine when something happened.