Hello!!!!
Which humanoid monster do you think acts as the best fit for the sin of pride?
I would imagine certain features for this hypothetical humanoid monster that best matches the sin of pride.
The lips are naturally inclined to pull down in a grimace of visceral disgust, perhaps bestial jaws like a feral dog's threatening posturing (or more fantastically, like lizard humanoid mercenaries from Kulipari). An exposed neck and upturned nose lean bird-like, but on the humanoid side, it leans more elven.
The more humanoid it is, the more aesthetically pleasing it is to us, thus the more we can view it as being vain about its appearance and beauty/handsomeness.
Care is taken for their appearance, which means they are either well-groomed (fur and hair), well-preened (feathers), or well-polished (scales). Long hair is very high maintenance, especially once it reaches really long lengths, so this or some fantasy equivalent for this humanoid monster would match. There are numerous equivalents. The obvious one is the manes of werelions. Plumes of feathers being a hair-equivalent can allow the being literally puff up like a peacock, and share in that symbolism of pride (angels, harpies, certain versions of sirens). Scales do not really have such hair equivalents, so you'd have to make something up or just give them hair.
However, you can look into aquatic humanoid fae for a truly helpful source of unique options. Hair-like gills (there is a modern equivalent with axolotls and salamanders, which is where this idea comes from for fantasy beings), jellyfish tentacle hair, elaborate series of fins, and plants adorning the head in alien, yet beautiful, styles of hair (see humanoid monsters such as nymphs, water sprites, naturalistic representations of mermaids and aquatic sirens, nixies).
Some Fae are known for being extremely prideful to an obsessive, eldritch degree that rivals even dragons. So in addition to their aquatic bretheren, the pixies, sprites, elves, and more classical fairies above the water line are ample sources for inspiration. I would recommend checking out the "Spiderwick Chronicles Field Guide: Fantastical World Around You."