Problem
There are a few bugs related to monsters' names, or 'n'aming them. These are the following.
1. Named non-uniques transformed with a PoEX don't keep their name, but cannot be re'n'amed under the pretense of "But the -foo- already has a name!" This only happens with monsters that are named by default, such as Kranach, Glod, Garth or royal guardians.
2. Royal guardians change names when they're animated as undead, and can't be 'n'amed. Their new forms probably shouldn't be initially named.
3. There's a rare bug that causes the same effect as problem number one, but without using a PoEX (at least with blink dogs).

How to reproduce it
1. Throw a potion of exchange at the aforementioned monsters, then try to 'n'ame them when they transform.
2. Find or wish for enough royal guardians until you get a corpse, then 'a'pply the Necromancy skill and note the results.
3. This one is hard to reproduce intentionally. So far, I've only seen it happen in the ID and with blink dogs (pretty much the only monsters I use the name command on).

Here's a savegame of a PC immediately next to a blink dog on I8. It is the first one ever generated during that game, hasn't been transformed with any potion, hasn't called for help, and has never been named by me before. Nonetheless, I'm unable to name it no matter what, getting "But the blink dog already has a name!" as a response.

Notes
See above.