  Given a universal elementary Chevalley-Demazure group $E_\Phi^{sc}(R)$ for
which its (standard) parabolic subgroups are finitely generated, we consider
the problem of classifying which parabolics $P(R) \subset E_\Phi^{sc}(R)$ are
finitely presented. We show that, under mild assumptions, this is equivalent to
the finite presentability of a suitable retract of $P$ which contains the Levi
factor. If the base ring $R$ is a Dedekind domain of arithmetic type, we
combine our results with well-known theorems due to Borel-Serre, Abels, Behr
and Bux to give a partial classification of finitely presentable $S$-arithmetic
subgroups of parabolics in split reductive linear algebraic groups.
