I've seen a lot of older films and TV shows were I was impressed with what they accomplished considering what little they had to work with.
Bingo. That is why part of my definition of "art" is the choices made by the creator based on time, materials and other resources. Everything about TOS has a theatrical, metaphorical atmosphere to it. So glitzy or über-realistic effects are not part of that picture. In fact, TOS is almost a radio show. Seriously, run any episode with your eyes closed and you will find that most action sequences can be followed because someone will fill you in verbally. "Direct hit amidships by photon torpedo. He's veering away."
The simple costume in "The Enemy Within" was enough to tell me "dog-like alien critter."