This "Non-Combat Pet Review" is made for Molten-WoW,
but supports all 3.3.5 WoW servers (private).
On retail (Blizzard servers/realms), it is only obtainable during the Hallow's End seasonal event as a drop from the Headless Horseman or with a [Treat Bag] or a [Crudely Wrapped Gift].
ON MOLTEN-WOW, it is available in the Points Shop for 5 vote points.
NOTE: It CAN be obtained the "normal way" as well.
Sinister Squashling does certainly not look bad, but doesn't go so well with most other than Halloween decorated environments (?+ Undercity?) and outfits. Its dance looks quite great, though (see video below).
"Its leg-like roots collapse and it looks like a Jack-o-lantern during its idle animation." - WoWWiki