Types of Anon
"Anon" can be broadly categorized into a few major versions of the concept:
Generic Human Anon
This covers a wide range of human "Anons" — these are usually bald human characters coloured in with paper-white or grey (less commonly with other colours), typically with an entirely featureless face. Artists may opt to give these Anon characters hair and clothing; in some cases Anons are depicted with limited facial features (such as a mouth and eyebrows), or simple cartoonish faces for emoting; less commonly, these characters are drawn with fully detailed facial features, but no eyes and grey/white skin, rarely, even normal human skin-colours are used (this is sometimes seen in hentai). While these Anons are frequently human males, human female anons may also be seen.
Anthro Anons are similar to the human anons above, acting as an anthropomorphic stand-in for the audience or as a generic character for a scene. They are rendered in a single flat tone or colour, and lack a fully drawn face. Anthro anons often tend to be generic canines. Unlike human anons, anthro anons are less likely to have entirely blank, featureless faces — they may have a simple face, or have an eyeless face, while the rest of their head is drawn in. They frequently have "You"/"YCH" etc. written on their body. Arguably the best & most well known Anon on the internet, 4chan Anon is a bald, featureless green skinned male, often shown wearing a suit and tie. This anon is intended to represent anonymous users of 4chan specifically, and is usually seen in art originally posted on 4chan, or made for a 4chan browsing audience. Similar to the generic human anon, these anons may have a mouth and/or eyebrows but often lack a nose and eyes, with crosshairs or a question mark possibly drawn in their place. 4chan anons may occasionally be depicted as wearing a skin-tight mask over their face, with realistic facial features "smoothed over". The Tumblr Anon is similar to the generic human anon but usually have a more rounded head, and are often shown wearing oversized sunglasses, evoking the default avatar on Tumblr. In some rare cases, a Tumblr anon may be anthrofied. Tumblr Anons are most often seen in response images from ask blogs. 
