update #1: wowhead has some nice 3D images of the possible cubs over here!
update #2: wowinsider has a screenshot of landro longshot offering the option to redeem a nightsaber cub. the post also discusses the theory of one or more of the datamined cubs being available as a digital (or even plush?) sales item via the blizzard PetStore in the future. (we'll have to wait and see)
update#3: there may be more than one winterspring cub in the works. judging from the 3D models wowhead has discovered, and the item and spell(s) for the winterspring cub, an actual vanity pet may be available as well as the mount quest item.
a few users over at WarcraftPets have already found possible locations for both the nightsaber cub and winterspring cub on the PTR.
the nightsaber cub is currently an option from landro longshot under "Twilight of the Dragons" on the test realms, so there's a high probability it's the common loot card from the next, next TCG expansion set (after "War of the Elements", which is releasing in march or april, i believe).
the winterspring cub, as one would expect, has been found in winterspring on the PTR as part of the frostsaber grind for the mount. it looks like the alliance version of the mount quest(s) has been updated to match the horde venomhide mount quest(s) in un'goro. currently, the winterspring cub doesn't seem to be a true vanity pet, though, and is only part of the mount grind and usable only in the questing zone.
it's still fairly early so things could change. we'll just have to wait and see!