Symbol for eye (looking for a library)
After checking user TrongVuong1998's answer, I went to the linked answer out of curiosity. And I discovered fontAwesome. So, here's my two cents to my own question:
\documentclass{standalone}
\usepackage{fontawesome}
\begin{document}
\fontsize{24}{0}\selectfont
\faEye
\end{document}
I have created two eyes symbols with Mathcha➡TikZ that reproduce largely your picture that they can be used in text and in math mode. Your names are \frontaleye and \lateraleye. With \scalebox{<factor>} you can increase or decrease the size of the symbols.
\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{tikz}
\usepackage{graphics}
\newcommand*\frontaleye{%
\scalebox{0.25}{
\tikzset{every picture/.style={line width=0.75pt}}
\begin{tikzpicture}[x=0.75pt,y=0.75pt,yscale=-1,xscale=1]
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (84.83,179.7) .. controls (84.92,169.21) and (100.66,160.85) .. (119.99,161.01) .. controls (139.32,161.18) and (154.92,169.81) .. (154.83,180.3) .. controls (152.72,170.91) and (137.95,163.54) .. (119.97,163.39) .. controls (101.99,163.23) and (87.1,170.35) .. (84.83,179.7) -- cycle ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (102.39,180.93) .. controls (102.39,171.24) and (110.25,163.39) .. (119.93,163.39) .. controls (129.62,163.39) and (137.47,171.24) .. (137.47,180.93) .. controls (137.47,190.61) and (129.62,198.47) .. (119.93,198.47) .. controls (110.25,198.47) and (102.39,190.61) .. (102.39,180.93) -- cycle ;
\draw [draw opacity=0][fill={rgb, 255:red, 255; green, 255; blue, 255 } ,fill opacity=1 ] (107,174) .. controls (107,171.24) and (109.24,169) .. (112,169) .. controls (114.76,169) and (117,171.24) .. (117,174) .. controls (117,176.76) and (114.76,179) .. (112,179) .. controls (109.24,179) and (107,176.76) .. (107,174) -- cycle ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (154.65,179.7) .. controls (154.65,190.58) and (139.02,199.41) .. (119.74,199.41) .. controls (100.46,199.41) and (84.83,190.58) .. (84.83,179.7) .. controls (85.71,190.15) and (101,198.47) .. (119.74,198.47) .. controls (138.48,198.47) and (153.77,190.15) .. (154.65,179.7) -- cycle ;
\end{tikzpicture}}
\,}
\newcommand*\lateraleye{%
\scalebox{0.15}{
\tikzset{every picture/.style={line width=0.75pt}}
\begin{tikzpicture}[x=0.75pt,y=0.75pt,yscale=-1,xscale=1]
\draw [line width=1.5] (300,100.33) .. controls (326,122) and (352,135) .. (378,139.33) .. controls (352,143.67) and (326,156.67) .. (300,178.33) ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (308.94,116.33) .. controls (313.87,116.33) and (317.86,125.51) .. (317.85,136.83) .. controls (317.84,148.15) and (313.84,157.33) .. (308.91,157.33) .. controls (303.99,157.32) and (300,148.14) .. (300.01,136.82) .. controls (300.02,125.5) and (304.02,116.32) .. (308.94,116.33) -- cycle ;
\draw [draw opacity=0][line width=1.5] (314.84,166.6) .. controls (311.87,164.64) and (309.14,162.18) .. (306.76,159.24) .. controls (295.12,144.82) and (296.6,124.33) .. (310.07,113.45) .. controls (311.48,112.32) and (312.96,111.33) .. (314.5,110.49) -- (331.14,139.55) -- cycle ; \draw [line width=1.5] (314.84,166.6) .. controls (311.87,164.64) and (309.14,162.18) .. (306.76,159.24) .. controls (295.12,144.82) and (296.6,124.33) .. (310.07,113.45) .. controls (311.48,112.32) and (312.96,111.33) .. (314.5,110.49) ;
\draw [fill={rgb, 255:red, 255; green, 255; blue, 255 } ,fill opacity=1 ] (304.43,124.2) .. controls (306.09,124.25) and (307.32,128.01) .. (307.18,132.6) .. controls (307.05,137.19) and (305.59,140.88) .. (303.93,140.83) .. controls (302.27,140.78) and (301.03,137.02) .. (301.17,132.43) .. controls (301.31,127.83) and (302.76,124.15) .. (304.43,124.2) -- cycle ;
\end{tikzpicture}
}\,}
\begin{document}
This is a \frontaleye frontal eye in text mode. In math-mode you have:
\[d\colon \frontaleye \to \mathbb{N}\]
Let \lateraleye text lateral eye in text mode and in math-mode:
\[r\colon \lateraleye \to \mathbb{Z}\]
\end{document}
You can see that it is possibile to choose the color of the eye.
\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{tikz}
\usepackage{graphics}
\newcommand*\frontaleye{%
\scalebox{0.25}{
\tikzset{every picture/.style={line width=0.75pt}}
\begin{tikzpicture}[x=0.75pt,y=0.75pt,yscale=-1,xscale=1]
\draw [draw opacity=0][fill={rgb, 255:red, 140; green, 196; blue, 74 } ,fill opacity=1 ] (104.5,179.99) .. controls (104.5,171.39) and (111.48,164.41) .. (120.08,164.41) .. controls (128.68,164.41) and (135.66,171.39) .. (135.66,179.99) .. controls (135.66,188.6) and (128.68,195.57) .. (120.08,195.57) .. controls (111.48,195.57) and (104.5,188.6) .. (104.5,179.99) -- cycle ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (84.83,179.7) .. controls (84.92,169.21) and (100.66,160.85) .. (119.99,161.01) .. controls (139.32,161.18) and (154.92,169.81) .. (154.83,180.3) .. controls (152.49,171.04) and (137.8,163.81) .. (119.97,163.66) .. controls (102.13,163.51) and (87.32,170.48) .. (84.83,179.7) -- cycle ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (113.6,179.91) .. controls (113.6,176.19) and (116.63,173.16) .. (120.36,173.16) .. controls (124.08,173.16) and (127.11,176.19) .. (127.11,179.91) .. controls (127.11,183.64) and (124.08,186.67) .. (120.36,186.67) .. controls (116.63,186.67) and (113.6,183.64) .. (113.6,179.91) -- cycle ;
\draw [draw opacity=0][fill={rgb, 255:red, 255; green, 255; blue, 255 } ,fill opacity=1 ] (195,141) .. controls (195,138.24) and (197.24,136) .. (200,136) .. controls (202.76,136) and (205,138.24) .. (205,141) .. controls (205,143.76) and (202.76,146) .. (200,146) .. controls (197.24,146) and (195,143.76) .. (195,141) -- cycle ;
\draw [fill={rgb, 255:red, 0; green, 0; blue, 0 } ,fill opacity=1 ] (154.65,179.7) .. controls (154.65,190.58) and (139.02,199.41) .. (119.74,199.41) .. controls (100.46,199.41) and (84.83,190.58) .. (84.83,179.7) .. controls (87.23,189.3) and (101.95,196.67) .. (119.74,196.67) .. controls (137.53,196.67) and (152.25,189.3) .. (154.65,179.7) -- cycle ;
\draw [draw opacity=0][fill={rgb, 255:red, 255; green, 255; blue, 255 } ,fill opacity=1 ] (125.45,172.24) .. controls (125.45,170.4) and (126.94,168.91) .. (128.78,168.91) .. controls (130.62,168.91) and (132.11,170.4) .. (132.11,172.24) .. controls (132.11,174.08) and (130.62,175.57) .. (128.78,175.57) .. controls (126.94,175.57) and (125.45,174.08) .. (125.45,172.24) -- cycle ;
\end{tikzpicture}
}\kern-.5em}
\begin{document}
This is a \frontaleye frontal colored eye in text mode and in math-mode
\[d\colon \frontaleye \to \mathbb{N}\]
\end{document}
If you use the dingbat package, you will get a frontal view of an eye by typing \eye.