Alien, Monster, Shifter... how do you define non-human characters?

For those who read SciFi / Fantasy / Paranormal romances, how do you define Alien vs Monster, when describing books? When do Shifters become Monsters?

Do you lump all non-humans together? Is there a very fine line between Aliens and Monsters that you have never exactly stated before, but have always known inside existed? Are all werewolves Monsters?

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This thought started rolling around in my brain when I read a "Monster-Fucking" article that Katee Robert, Ruby Dixon, and Opal Reyne were all quoted in. I love Ruby Dixon, but I realized I would not consider her to be a Monster Romance author. In my brain, Ruby writes Alien romance, and that is very different from the Paranormal/Monster (depending on the book) written by Katee Robert or the monsters that Opal Reyne writes.

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So here's my random musings on the difference between them:

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• Alien - must exist outside off-planet Earth, if they are bipedal and walk on two feet, with a vaguely humanoid appearance they are aliens. Ice Planet Barbarians are the perfect example. 100% Alien, 0% Monster. Zoey Draven's Horde Kings and Lizzy Bequin's Ukkurs are Aliens in my mind as well. But there are definitely some non-Earth characters that I would label a Monster rather than an Alien, like Octavia Hyde's Nepenthe, or SJ Sanders' Ragoru.
• Monster - less human appearing than Aliens, usually on Earth. Werewolves who are permanently in a wolf form (Muscles & Monsters by Ashley Bennet or Ensnared by the Werewolf by Lillian Lark), Opal Reyne's duskwalkers, Finley Fenn's orcs, Kathryn Moon's demons, any character with tentacles, all Monsters in my brain.
• Shifter - spend most of the book in their human forms, and their other forms are a "standard" animal. Suzanne Wright's series are here, as well as Thea Harrison's Wyre characters, and Heather Guerre's werewolves.

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But where do characters like Regine Abel's land? Is "I Married A Lizardman" an Alien romance or a monster one? What about "I Married a Naga"? Snake-characters feel very Monster to me, but I think I would still classify them as an Alien romance? But looking at very similar characters, Lillian Lark's Basilisks, who shift between human and snakeform but spend a lot of the time in human form, I consider Monster, not Shifters (even though I am now contradicting my own definition 🤷🏻‍♀️)

Interested to hear what others think!