From Wikipedia, “A practical effect is a special effect produced physically, without computer-generated imagery or other post-production techniques.”
Some of my favorite practical effect species are from TOS. There’s the Horta from TOS “The Devil in the Dark” s1e25. I was about 9 years old when I first saw the Horta episode. Old enough to know it wasn’t real, but young enough to be utterly fascinated by something that looked like an angry chunk of hot lava making its way around on the ground.
TOS’s the Salt Vampire’s look scared little kid me. Truth be told, the way it looks still gives me the creeps.
The Gorn (TOS “Arena” s1e18) didn’t scare me, but I thought it was intimidating. Sure, it looks like somebody in a lizard man suit. But, that Gorn had a knife! And, was trying to kill Kirk! Bonus points for the Gorn’s sparkly, stylish armor.
I think tribbles are just adorable.
I have to say that even as a schoolchild neither the TOS Gorn nor the Horta were truly frightening.
I was watching monster movies on after school television, so the costumes for the Gorn and Horta were obviously that. I loved the Horta anyway.
The Gorn just seemed so inferior as compared to the Creature of the Black Lagoon and other contemporary examples. Time has only made the rubber suit more silly. In my view, the creature in TNG Conspiracy is falling into the same category now - memeable but not really scary.
But the Salt Monster I can agree was very disturbing. The fact that it was only shown briefly and there was no option for playback likely magnified it’s impact.
In terms of my own favourite, Andorians in Enterprise for the animatronics.
I like the way the Andorian antennae reflected their mood.