Instructions
Open up your player inventory by pressing the "B" key or by clicking on your bags in the lower right-hand corner of the screen.
Locate the Sinister Squashling item and right-click on it to teach your character how to summon it. The item should vanish from your inventory.
Close your inventory and open your "Companions" window by pressing the "P" key and then selecting the "Companions" tab at the bottom of the window that will appear. All of the vanity pets you have acquired will be listed in the bottom half of this window and the upper half will display the pet you currently have selected.
Locate the Sinister Squashling pet in the list of vanity pets. If it was the most recent pet you acquired, it will be at the end of the list. Look for an image of a jack-o-lantern.
Click on the Sinister Squashling icon and then press the "Summon" button in the display window beneath the pet's name. You can also right-click on the icon to summon it.