Refresh My Memory #2

Here's another half-remembered horror tale I'm hoping you can help me remember:

The story is about a little boy with a beloved pet...a beloved pet blob. The creature is named "Spot" or "Fido" or something equally innocuous. Everything's fine until one day when the blob starts eating people. (I remember specifically that it got the Avon Lady when she rang the doorbell.) The boy is informed that his pet will have to be destroyed (just like Old Yeller), but he won't hear of it. The boy finally finds the blob (who had run away) and breaks down in tears. The boy's father joins him and assures him that everything's all right: "Sometimes pets just get a little touchy...just before they have puppies!" And the father holds out a handful of writhing mini-blobs.

I'm pretty sure the story was illustrated by either Richard Corben or Berni Wrightson. (Not that they're similar, but they're both masters of the kind of grotesque imagery featured in this story.)

So, anybody else remember this?