  We report measurements of weakly nonlinear electronic transport, as measured
by third-harmonic voltage generation $V_{3\omega}$, in the low-carrier density
semimetallic ferromagnet EuB$_6$, which exhibits an unusual magnetic ordering
with two consecutive transitions at $T_{c_1} = 15.6$\,K and $T_{c_2} =
12.5$\,K. Upon cooling in zero magnetic field through the ferromagnetic
transition, the dramatic drop in the linear resistivity at the upper transition
$T_{c_1}$ coincides with the onset of nonlinearity, and upon further cooling is
followed by a pronounced peak in $V_{3 \omega}$ at the lower transition
$T_{c_2}$. Likewise, in the paramagnetic regime, a drop of the material's
magnetoresistance $R(H)$ precedes a magnetic-field-induced peak in nonlinear
transport. A striking observation is a linear temperature dependence of
$V_{3\omega}^{\rm peak}(H)$. We suggest a picture where at the upper transition
$T_{c_1}$ the coalescing MP form a conducting path giving rise to a strong
decrease in the resistance. The MP formation sets in at around $T^\ast \sim
35$\,K below which these entities are isolated and strongly fluctuating, while
growing in number. The MP then start to form links at $T_{c_1}$, where
percolative electronic transport is observed. The MP merge and start forming a
continuum at the threshold $T_{c_2}$. In the paramagnetic temperature regime
$T_{c_1} < T < T^\ast$, MP percolation is induced by a magnetic field, and the
threshold accompanied by charge carrier delocalization occurs at a single
critical magnetization.
