New frostwyrm for the current arena season has already been announced some weeks ago so those armored ones shouldn't be for pvpers.If you didn't notice, the four pics don't show the same armored frostwyrm, their colors are slightly different like the pvp's one.So, it doesn't seem to be a drop or reward for any kind of hard mode ( or there would have been only 2 for respectively 10 and 25)but most likely something where we will have the choose. At least I hope it will happen in this way =)
PS sorry for my bad english