Actually, it's an aggro'd enemies list so if something has no aggro on you, it won't appear even if other members have it. Spiny, not sure since my memory is hazy, puts aggro on all members but only attracts to one with 100k aggro which is why it shows up in the list but when you run dungeons and have zero aggro on a mob, it won't show up.
To the poster below, that's why I said spiny is special case and pointed out it might not work with normal mobs.