  Crystal and magnetic structures of the $x=0.2$ member of La$_{\rm
0.8-x}$Tb$_{\rm x}$Ca$_{0.2}$CoO$_3$ perovskite series have been determined
from the powder neutron diffraction. Enhancement of the diffraction peaks due
to ferromagnetic or cluster glass ordering is observed below $T_C=55$ K. The
moments evolve at first on Co sites, and ordering of Ising-type Tb$^{3+}$
moments is induced at lower temperatures by a molecular field due to Co ions.
The final magnetic configuration is collinear F$_x$ for cobalt subsystem, while
it is canted F$_x$C$_y$ for terbium ions. The rare-earth moments align along
local Ising axes within \textit{ab}-plane of the orthorhombic $Pbnm$ structure.
The behavior in external fields up to $70-90$ kOe has been probed by the
magnetization and heat capacity measurements. The dilute terbium ions
contribute to significant coercivity and remanence that both steeply increase
with decreasing temperature. A remarkable manifestation of the Tb$^{3+}$ Ising
character is the observation of a low-temperature region of anomalously large
linear term of heat capacity and its field dependence. Similar behaviours are
detected also for other terbium dopings $x=0.1$ and 0.3.
