The bug
Leads are rendered shifted upwards based on the eye level of the leashed entity. This causes the lead start to be rendered too high for taller mobs like iron golems or horses.
Code analysis
Based on 1.12 decompiled using MCP 9.40 PRE 1
The method net.minecraft.client.renderer.entity.RenderLiving.renderLeash(T, double, double, double, float, float)
shifts the rendering position upwards based on the leashed entity's eye level instead of only the end position, which might be partwise the cause for MC-13823.
Linked issues
is duplicated by 9
relates to 2
Attachments
Comments 16
Confirmed in 13w21b.
still in 1.6.1
Confirm 14w07a
Confirmed for 1.13-pre5
Confirmed for 1.13.1.

confirmed for minecraft 1.14 pre-release 1

Confirmed for 1.14.1 release.
Confirmed in 1.16 Pre-release 2.
I can confirm this issue for snapshot 13w19a.