Earlier that day Tracy received delivery of a parcel for Miss Cranshaw, who wasn't at home. Her parents are away as well, possibly overnight. Tracy is to hold onto the package until Miss Cranshaw returns.
Of course this wouldn't be a horror story if her curiosity didn't get the better of her. She carefully opens the little box...
For a second or two she stared at the horrible little dried-up thing in her hand. Then shuddering with revulsion, she flung it away and ran back to the kitchen. ...It had been alive once, but Tracy had never even imagined an animal like that, not in her most awful nightmares.
After gathering her senses she sets about putting Miss Cranshaw's package back together, but the little dead thing is nowhere to be found. Panicking, she scours the house for it only to be interrupted by someone at the door. Miss Cranshaw! Tracy denies having received any package, but Miss Cranshaw doesn't seem convinced.
"This package," said Miss Cranshaw, speaking slowly and distinctly, "has a very special pet inside. It needs certain kind of care. If it is not treated correctly, it can be fatal."
As night falls, Tracy does find the creature...or more correctly, it finds her. Seems the little beast has been steadily growing until it is nearly as large as she, and is now sniffing and scrabbling around the basement, hunting her...
This story, by Rita Ritchie, comes from Baleful Beasts and Eerie Creatures, and is probably my second favorite to The Patchwork Monkey. And again, we do not have the contrived happy ending so common in children's horror.