> > vonfola (vánfoli ?)
>
> Presumably so. I don't know the textual history of this word here, but
> CV refers 'vánfoli' to 'váfoli', 'váfolald' "a vicious horse" [
> http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0684.png ] - top of
> left column. Only that seems to clash with his 'óreyndur' explanation.

In the mid 19th century, George W. Dasent translated the word as "a
dark horse", noting "The Icelandic 'vanfoli' or 'váfoli' answers
exactly to this sporting term. The horse was four years old, but Mord
called it an untried foal" (The Story of Burnt Njal 1861, p. 505).

Ludwig Chr. Müller has 'vanfoli' "daarlig fole", a poor foal (Islandsk
læsebog 1837, p. 420).

I found nothing in the Orðabók Háskólans ritmálsskrá for 'vonfoli',
'vánfoli', 'vanfoli' or 'váfoli'. The Orðabók Háskólans recognised
'vonfoli', but this was the only example. It didn't recognise
'vánfoli' or 'váfoli' and the website stopped working before I could
get to 'vanfoli'.

OED: "dark horse (Racing slang), a horse about whose racing powers
little is known; hence fig. a candidate or competitor of whom little
is known or heard, but who unexpectedly comes to the front. In U.S.
Politics, a person not named as a candidate before a convention, who
unexpectedly receives the nomination, when the convention has failed
to agree upon any of the leading candidates."