A list of all the built-in EdtiorGUI icons in Unity. Use EditorGUIUtility.IconContent([icon name]) to access them.

UnityEditorIcons.txt

ScriptableObject Icon
_Popup
_Help
Clipboard
SocialNetworks.UDNOpen
SocialNetworks.Tweet
SocialNetworks.FacebookShare
SocialNetworks.LinkedInShare
SocialNetworks.UDNLogo
animationvisibilitytoggleoff
animationvisibilitytoggleon
tree_icon
tree_icon_leaf
tree_icon_frond
tree_icon_branch
tree_icon_branch_frond
editicon.sml
TreeEditor.Refresh
TreeEditor.Duplicate
TreeEditor.Trash
TreeEditor.AddBranches
TreeEditor.AddLeaves
TreeEditor.Trash
preAudioPlayOn
preAudioPlayOff
AvatarInspector/RightFingersIk
AvatarInspector/LeftFingersIk
AvatarInspector/RightFeetIk
AvatarInspector/LeftFeetIk
AvatarInspector/RightFingers
AvatarInspector/LeftFingers
AvatarInspector/RightArm
AvatarInspector/LeftArm
AvatarInspector/RightLeg
AvatarInspector/LeftLeg
AvatarInspector/Head
AvatarInspector/Torso
AvatarInspector/MaskEditor_Root
AvatarInspector/BodyPartPicker
AvatarInspector/BodySIlhouette
Mirror
SpeedScale
Toolbar Minus
Toolbar Plus More
Toolbar Plus
AnimatorController Icon
TextAsset Icon
Shader Icon
boo Script Icon
cs Script Icon
js Script Icon
Prefab Icon
Profiler.NextFrame
Profiler.PrevFrame
sv_icon_none
ColorPicker.CycleSlider
ColorPicker.CycleColor
EyeDropper.Large
ClothInspector.PaintValue
ClothInspector.ViewValue
ClothInspector.SettingsTool
ClothInspector.PaintTool
ClothInspector.SelectTool
WelcomeScreen.AssetStoreLogo
WelcomeScreen.AssetStoreLogo
AboutWindow.MainHeader
UnityLogo
AgeiaLogo
MonoLogo
Toolbar Minus
PlayButtonProfile Anim
StepButton Anim
PauseButton Anim
PlayButton Anim
PlayButtonProfile On
StepButton On
PauseButton On
PlayButton On
PlayButtonProfile
StepButton
PauseButton
PlayButton
ViewToolOrbit On
ViewToolZoom On
ViewToolMove On
ViewToolOrbit On
ViewToolOrbit
ViewToolZoom
ViewToolMove
ViewToolOrbit
ScaleTool On
RotateTool On
MoveTool On
ScaleTool
RotateTool
MoveTool
PauseButton
PlayButton
Toolbar Minus
Toolbar Plus
UnityLogo
_Help
_Popup
Icon Dropdown
Avatar Icon
AvatarPivot
SpeedScale
AvatarInspector/DotSelection
AvatarInspector/DotFrameDotted
AvatarInspector/DotFrame
AvatarInspector/DotFill
AvatarInspector/RightHandZoom
AvatarInspector/LeftHandZoom
AvatarInspector/HeadZoom
AvatarInspector/RightLeg
AvatarInspector/LeftLeg
AvatarInspector/RightFingers
AvatarInspector/RightArm
AvatarInspector/LeftFingers
AvatarInspector/LeftArm
AvatarInspector/Head
AvatarInspector/Torso
AvatarInspector/RightHandZoomSilhouette
AvatarInspector/LeftHandZoomSilhouette
AvatarInspector/HeadZoomSilhouette
AvatarInspector/BodySilhouette
Animation.AddKeyframe
Animation.NextKey
Animation.PrevKey
lightMeter/redLight
lightMeter/orangeLight
lightMeter/lightRim
lightMeter/greenLight
Animation.AddEvent
SceneviewAudio
SceneviewLighting
MeshRenderer Icon
Terrain Icon
sv_icon_none
BuildSettings.SelectedIcon
Animation.AddEvent
Animation.AddKeyframe
Animation.NextKey
Animation.PrevKey
Animation.Record
Animation.Play
PreTextureRGB
PreTextureAlpha
PreTextureMipMapHigh
PreTextureMipMapLow
TerrainInspector.TerrainToolSettings
TerrainInspector.TerrainToolPlants
TerrainInspector.TerrainToolTrees
TerrainInspector.TerrainToolSplat
TerrainInspector.TerrainToolSmoothHeight
TerrainInspector.TerrainToolSetHeight
TerrainInspector.TerrainToolRaise
SettingsIcon
PauseButton
PlayButton
PreMatLight1
PreMatLight0
PreMatTorus
PreMatCylinder
PreMatCube
PreMatSphere
PreMatLight1
PreMatLight0
Camera Icon
Toolbar Minus
Toolbar Plus
Animation.EventMarker
AS Badge New
AS Badge Move
AS Badge Delete
WaitSpin00
WaitSpin01
WaitSpin02
WaitSpin03
WaitSpin04
WaitSpin05
WaitSpin06
WaitSpin07
WaitSpin08
WaitSpin09
WaitSpin10
WaitSpin11
WelcomeScreen.AssetStoreLogo
WelcomeScreen.UnityAnswersLogo
WelcomeScreen.UnityForumLogo
WelcomeScreen.UnityBasicsLogo
WelcomeScreen.VideoTutLogo
WelcomeScreen.MainHeader
UnityLogo
preAudioPlayOn
preAudioPlayOff
Animation.EventMarker
PreMatLight1
PreMatLight0
PreMatTorus
PreMatCylinder
PreMatCube
PreMatSphere
TreeEditor.Duplicate
Toolbar Minus
Toolbar Plus
PreTextureMipMapHigh
PreTextureMipMapLow
PreTextureRGB
PreTextureAlpha
VerticalSplit
HorizontalSplit
Icon Dropdown
PrefabNormal Icon
PrefabModel Icon
PrefabNormal Icon
Prefab Icon
GameObject Icon
preAudioLoopOn
preAudioLoopOff
preAudioPlayOn
preAudioPlayOff
preAudioAutoPlayOn
preAudioAutoPlayOff
BuildSettings.Web.Small
BuildSettings.Standalone.Small
BuildSettings.iPhone.Small
BuildSettings.Android.Small
BuildSettings.BlackBerry.Small
BuildSettings.Tizen.Small
BuildSettings.XBox360.Small
BuildSettings.XboxOne.Small
BuildSettings.PS3.Small
BuildSettings.PSP2.Small
BuildSettings.PS4.Small
BuildSettings.PSM.Small
BuildSettings.FlashPlayer.Small
BuildSettings.Metro.Small
BuildSettings.WP8.Small
BuildSettings.SamsungTV.Small
BuildSettings.Web
BuildSettings.Standalone
BuildSettings.iPhone
BuildSettings.Android
BuildSettings.BlackBerry
BuildSettings.Tizen
BuildSettings.XBox360
BuildSettings.XboxOne
BuildSettings.PS3
BuildSettings.PSP2
BuildSettings.PS4
BuildSettings.PSM
BuildSettings.FlashPlayer
BuildSettings.Metro
BuildSettings.WP8
BuildSettings.SamsungTV
TreeEditor.BranchTranslate
TreeEditor.BranchRotate
TreeEditor.BranchFreeHand
TreeEditor.BranchTranslate On
TreeEditor.BranchRotate On
TreeEditor.BranchFreeHand On
TreeEditor.LeafTranslate
TreeEditor.LeafRotate
TreeEditor.LeafTranslate On
TreeEditor.LeafRotate On
sv_icon_dot15_pix16_gizmo
sv_icon_dot1_sml
sv_icon_dot4_sml
sv_icon_dot7_sml
sv_icon_dot5_pix16_gizmo
sv_icon_dot11_pix16_gizmo
sv_icon_dot12_sml
sv_icon_dot15_sml
sv_icon_dot9_pix16_gizmo
sv_icon_name6
sv_icon_name3
sv_icon_name4
sv_icon_name0
sv_icon_name1
sv_icon_name2
sv_icon_name5
sv_icon_name7
sv_icon_dot1_pix16_gizmo
sv_icon_dot8_pix16_gizmo
sv_icon_dot2_pix16_gizmo
sv_icon_dot6_pix16_gizmo
sv_icon_dot0_sml
sv_icon_dot3_sml
sv_icon_dot6_sml
sv_icon_dot9_sml
sv_icon_dot11_sml
sv_icon_dot14_sml
sv_label_0
sv_label_1
sv_label_2
sv_label_3
sv_label_5
sv_label_6
sv_label_7
sv_icon_none
sv_icon_dot14_pix16_gizmo
sv_icon_dot7_pix16_gizmo
sv_icon_dot3_pix16_gizmo
sv_icon_dot0_pix16_gizmo
sv_icon_dot2_sml
sv_icon_dot5_sml
sv_icon_dot8_sml
sv_icon_dot10_pix16_gizmo
sv_icon_dot12_pix16_gizmo
sv_icon_dot10_sml
sv_icon_dot13_sml
sv_icon_dot4_pix16_gizmo
sv_label_4
sv_icon_dot13_pix16_gizmo

Thanks!.

https://unitylist.com/p/5c3/Unity-editor-icons

shows all icons with code names

Turned this into a utility here: https://gist.github.com/ChristopherSchubert/9f2427f190e2ea611450e0019d349bbf

I made a new tool inspired by @nukadelic source code : https://github.com/nukadelic/UnityEditorIcons
https://github.com/KumoKyaku/Megumin.Explosion#%E5%8A%9F%E8%83%BD%E5%88%97%E8%A1%A8
Megumin.UnityPackage/Packages/megumin.explosion4unity/Editor/Scripts/EditorIcon/SetScriptIcon.cs

I made a list with icons displayed https://github.com/jasursadikov/unity-editor-icons

"FindTexture" It seems that it is no longer valid in unity2018 or later,Use "EditorGUIUtility.IconContent"

FindTexture is working fine on 2021.3.6f1 so far. EditorGUIUtility.IconContent also works, but can't find some assets listed. One of them being "UnityLogo".