Watching all this I thought, early on, they're going to put us through all this and never show us the damn snake, or blob, or whatever, are they??!! After all the boredom and straight-line simplicity of that storyline (really disappointing after 1897, which had complexity and overlapping storylines enough to please anyone) , a glimpse is the least they could have done.
They didn't always trip up as regards the props and effects. The Hand was very well done, and left more of a lasting impression with me as a kid, than anything else on the program. Barnabas's old age mask was so incredible that they brought it out again for one of the large-screen films. (The only time I've ever seen convincing old-age makeup, anywhere.) What they could have done was to build a portion of the creature, a really unexpected-looking, startling small portion, and have it occupy just a bit of the screen, as we look past this to see at the same time a human's horrified response, all taking place in the dim light of the Room, so any flaws are hidden, and the mystery quotient is dialed up. (I'm getting verbose today, sorry)
Sure, it's great to leave things to the imagination, but the imagination needs to have a little something to work with. An effective, horrifying image such as this might even have inspired the writers to come up with better material. Their imaginations need something to spark them, too. Asthma and a locked door over and over wouldn't inspire me, I know.