Here's the use text. It's a little more than just the familiar hatching:
You put the Crimbo sapling in your Familiar-Gro™ Terrarium.
The sapling bursts immediately into a full-grown Crimbo shrub, complete with ornaments and lights. "IIIIIIT'S CRIMBO TIIIIIME!" he shouts ecstatically.
You decide to name him Troddeus.You suddenly remember an old box of Crimbo decorations you've been keeping in the attic. The... attic of your closet. You find a stepladder and go grab them. You acquire an item: box of old Crimbo decorations