This is almost certainly a reference to the recent Druid Travel form bug (hotfixed soon after the release of Patch 5.0.4), where Stag form Druids could mount other Stag form Druids, forming a Stag Stack that could reach up to the sky.
Video of the old bug: http://www.youtube.com/watch?v=H-hNYKyF8FA
Players do not get to control where they place the stag. I used the item and the stag appeared on the ground on my right side. The "your Stackable Stag will stack on top of it" part isn't always accurate. I'll need to play around with the item more, but it seems that the stag will not stack on a nearby stag if you, yourself, already have a summoned stag present.
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