By beating the first level, at the beginning of the second one you will find the world map on the floor, unlocking it

First get the ghost lantern, which I found by beating the bat boss level on heroic (need confirmation that this is the method), then tap on a unmarked graveyard on the world map, above the bat boss level. You will then get the ghost pet.

Update: upon finding the ghost lantern, you can interact with a unmarked graveyard on the world map, above the bat boss. Doing so will unlock the Ghost pet

Not entirely sure what the details are, but I got the lantern by completing the bat boss level in world map on the more difficult setting, hope it helps