In those long ago days when I was new to D&D, and every creature and magic item was still mysterious and wonderful, one of my favorite monsters was the owl bear. This was due almost entirely to the Keep on the Borderlands module, particularly the illustration at the very beginning of two fighters and a halfling battling the beast.
I think on some level, I just assumed that "owl bear" was not to be taken literally, much like a catfish is not literally a fish with a cat's head, but a fish that has a few features reminiscent of felines, or a dragonfly is a flying insect reminiscent of a dragon, not literally a hybrid of dragon and housefly.
Gnolls are another of those monsters whose description I never took literally, but in their case, the illustrators did. "Gnolls are beings of low intelligence that appear to be human-like hyenas." I imagined them looking like hyenas in the same way that a person described as having a horse face looks like a horse or a mousy person resembles a small rodent: Having some features that evoke the impression of the animal without actually being identical. Fortunately, the Basic rulebook offered no illustrations of gnolls. It wasn't until much later, when I saw the entry in the 2nd Edition AD&D Monstrous Manual, that I was introduced to the idea of the gnoll as a mean furry, which is significantly less intimidating than what I had in mind. Apparently the idea of a gnoll being a magically-bred gnome/troll hybrid had been dropped by that point, being totally irreconcilable with such a silly illustration.
Orcs must have gone through a similar evolution. Somehow they went from the rulebook description of "ugly human-like creatures who look like a combination of animal and man" to a race of green-skinned mutant pig-men. My mental image of them had always been basically very ugly humans with feral features and animal traits that were non-specific but decidedly more canine than porcine in aspect. In that I was almost certainly influenced by Gygax's reference to "dog-men" in the rumors table of B2. I imagined them with skin tones of yellow and burnt orange, or fish-belly pale beneath coarse, wiry hair, but never green.
I'd venture to say that most of my ideas of what monsters look like came from the old school rule books and modules. Say what you want about the simplicity of those drawings, but I've never yet seen a troglodyte or a kobold that really fired my imagination like the ones in the Moldvay Basic Rules set did. Not only did they give me a clear idea of what those monsters looked like, they served as a stylistic prompt to conjure the mental images of the many monsters that were represented only by text without pictures. You'd think that having color illustrations of each and every monster would be a good thing, but in my experience the opposite was true. The 2nd Edition AD&D books, in which that had become standard practice, left me cold by comparison. They never quite measured up to the monsters in my mind.