The story
- Proto-Indo-European*kʷelh₁-redup.
- Proto-Indo-European*kʷekʷléh₂
- Proto-Germanic*hweulō
- Old Englishhwēol
- Middle Englishwhel
- Englishwheel
- Proto-Indo-European*ḱers-
- Proto-Indo-European*ḱr̥sós?
- Proto-Indo-European*h₁wers-?
- Proto-Indo-European*wers-
- Proto-Indo-European*wérsēn
- Proto-Indo-Iranian*(w)ŕ̥šā
- Proto-Iranian*(w)ŕ̥šāder.?
- Proto-Germanic*hrussą
- Proto-West Germanic*hross
- Old Englishhors
- Middle Englishhors
- Englishhorse
- English todaywheelhorse
From wheel + horse. Sense 2 (“person who labors heavily for a cause”) is from the fact that a horse nearest to the wheels of a carriage that it is drawing does much of the pulling work.
Etymology from Wiktionary, CC BY-SA 4.0.