  Rapidly evolving transients, or objects that rise and fade in brightness on
timescales two to three times shorter than those of typical Type Ia or Type II
supernovae (SNe), have uncertain progenitor systems and powering mechanisms.
Recent studies have noted similarities between rapidly evolving transients and
Type Ibn SNe, which are powered by ejecta interacting with He-rich
circumstellar material (CSM). In this work we present multiband photometric and
spectroscopic observations from Las Cumbres Observatory and Swift of four
fast-evolving Type Ibn SNe. We compare these observations with those of rapidly
evolving transients identified in the literature. We discuss several common
characteristics between these two samples, including their light curve and
color evolution as well as their spectral features. To investigate a common
powering mechanism we construct a grid of analytical model light curves with
luminosity inputs from CSM interaction as well as $^{56}$Ni radioactive decay.
We find that models with ejecta masses of $\approx 1-3$ M$_\odot$, CSM masses
of $\approx 0.2-1$ M$_\odot$, and CSM radii of $\approx 20-65$ au can explain
the diversity of peak luminosities, rise times, and decline rates observed in
Type Ibn SNe and rapidly evolving transients. This suggests that a common
progenitor system$-$the core collapse of a high-mass star within a dense CSM
shell$-$can reproduce the light curves of even the most luminous and
fast-evolving objects, such as AT 2018cow. This work is one of the first to
reproduce the light curves of both SNe Ibn and other rapidly evolving
transients with a single model.
