I really love the difference between the Demogorgon in 1 and the Demo's in later seasons!
The Demogorgon is a core staple of the show, and personally, i feel like introducing more humanoid lifeforms would've made things... a bit too fantasy feeling. I think keeping it to one single main humanoid species is an important part of what made this show work so well, and it's clear the writer's all agree (that said i love the Demobats, and the Vine's technically count as a lifeform/entity ;D).
But that decision raises an issue - keeping it interesting. Season 2 did this wonderfully - Demodogs!
Not only is there a narrative purpose to the Demodogs (to prevent spamming adult Demos and ruining the impact of the S1 Demo), there's a in-universe purpose. They're at a developmental stage where they're quadrupedal, faster, work in packs, and are far better for guarding and clearing rooms/small areas as opposed to doing physical work and completing objectives or acting as pure brute force like an adult Demo would. They're also seemingly in a sort of limbo where they're permanently stuck as Juveniles - likely due to the Particles nesting inside of them being able to directly enhance or change their bodies - especially seeing as Billy/The Flayed were far, far stronger than they naturally would be and could survive things no normal human can. I'd imagine MF particles could prevent a host from aging as well.
4 touches on the Demo's in a cool way as well that also sets up 5's use of Demo's in a way. The captive Demo in Russia is completely disconnected from the Hivemind, but it's nothing like the Season 1 Demo - It's a strong, brutal, agile entity EASILY able to kill multiple armed people and that's specifically because it's both captive, but also being trained for use in combat. This shows us that, when trained for it, Demogorgon's can be more than the slow methodical predator the Season 1 Demogorgon was - they can be agile, strong, and great at killing.
And that sets up 5 in an awesome way. The Demogorgon's in 5 are specifically designed for killing - Vecna's plan was to smash The Abyss into our world, and that'd require force, in our dimension specifically.
The Season 1 Demo's overall operational objective was to establish infrastructure in the Upside Down (the nest in the library), breach out into our world and look for potential victims for Vecna, and bring them back to him. That's it. It wasn't meant to kill - it was born in The Abyss under natural circumstances and was chilling feeding on an egg when it was transported into the Upside Down upon it's creation - and it was allowed to feed and hunt when it wasn't needed. But the Demo's in 5 are different. They were born specifically to kill. They were very likely physically manipulated by the particles inside of them to be stronger than they naturally are like The Flayed.
The S1 Demo was essentially a natural born predator used as a drone whereas the Demo's in 5 were soldiers born to slash and gash. That's how i'd sum it up lol ;D
I love how the Demo's feel cool, unique, and fresh in each season they're in. And i also love how it's written in a way where the S5 Demo's can be fast, agile, and strong without erasing the S1 Demo being slow and methodical. S5 Demo's were born and deployed to kill, the S1 Demo was a natural predator born in The Abyss under natural circumstance. They both make sense at the same time!
Here's another small thing i love: how the Demo's, despite Hivemind manipulation, sometimes still act as they do naturally. Specifically the S5 Demo that went up against Karen. It acted very similarly to the S1 Demo - slow, calculated, intimidating. It didn't run around and throw her to the side to immediately get to Holly. The S4 Demo did the same at times - they love to slowly walk over and tower over their victims even when they can get a good shot in. They're predators, they want to feel big, even when they have a objective to complete.