  MRT unstable plumes are observed in a loop-like eruptive prominence using
SDO/AIA observations. The small-scale cavities are developed within the
prominence, where perturbations trigger dark plumes (P1 \& P2) propagating with
the speed of 35-46 km s$^{-1} $. The self-similar plume formation shows
initially the growth of linear MRT unstable plume (P1), and thereafter the
evolution of non-linear single mode MRT unstable second plume (P2). The
Differential Emission Measure (DEM) analysis shows that plumes are less denser
and hotter than the prominence. We have estimated the observational growth rate
for both plumes as 1.32$\pm$0.29$\times$ 10$^{-3}$ s$^{-1}$ and
1.48$\pm$0.29$\times$ 10$^{-3}$ s$^{-1}$ respectively, which are comparable to
the estimated theoretical growth rate (1.95$\times$ 10$^{-3}$ s$^{-1}$) . The
nonlinear phase of an MRT unstable plume (P2) may collapse via Kelvin-Helmholtz
vortex formation in the downfalling plasma. Later, a plasma thread has been
evident in the rising segment of this prominence. It may be associated with the
tangled field and Rayleigh-Taylor instability. The tangled field initiates
shearing at the prominence-cavity boundary. Due to this shear motion, the
plasma downfall has occurred at the right part of the prominence-cavity
boundary. It triggers the characteristic of KH unstable vortices and MRT
unstable plasma bubbles propagating at different speeds and merging with each
other. The shear motion and lateral plasma downfall may initiate hybrid KH-RT
instability there.
