No, Miguel, Sergei is right. Masculines in -ch (*-x) had plurals in -
szy (-*s^i) in Old Polish, e.g. <mnich> 'monk', <mniszy>. Forms like
<mnichy> or <mnisi> are of recent origin (<-si> did not become
widespread until the 17th century).
Old masculine first names in -o (e.g. Mieszko, the first historical
ruler of Poland, Jas'ko [dim. of Jan]), as well as recent diminutives
like Jasio have the same masculine-type case forms as the
corresponding o-less bases (gen.sg. Mieszka, Jas'ka, Jasia, etc.).
Family names like Kos'ciuszko or Jagiel/l/o are of eastern origin (in
these cases, Belarusian and Lithuanian, respectively). They are
declined like *-a-nouns (which is sometimes might seem to reflect
their etymology, e.g. Lith. Jogaila). So is the nursery word
<tato> 'dad' (gen. <taty>). However, this usage is also quite recent.
Before the 17th century the genitive of such names ended in <-a>, the
instrumental in <-em>, etc. Archaic forms like <Kos'ciuszka,
Kos'ciuszkowi, Kos'ciuszkiem> occurred sporadically as late as the
19th century.
Piotr
--- In cybalist@..., Miguel Carrasquer Vidal <mcv@...> wrote:
> Old Polish. Modern Pol. would be **prusi, **pruchów (it is in fact
> Prusacy, Prusaków, The country name is Prusy, Prus [pl.]).
>
> BTW, Piotr, why are names in -o declined the way they are
> (Kos'ciuszko, Kos'ciuski, etc.)???