  An extended HI cloud (VIRGOHI 21) with an HI mass of $\sim10^8$ M$_{\odot}$
and no apparent optical counterpart was recently discovered in the Virgo
cluster. In order to understand the origin of physical properties of apparently
isolated HI clouds like the VIRGOHI21, we numerically investigate physical
properties of tidal HI debris that were formed by galaxy-galaxy interaction in
clusters of galaxies. Our hydrodynamical simulations demonstrate that tidal
debris with total HI masses of $10^8-10^9$ M$_{\odot}$ can have (1) wide HI
velocity widths ($>200$ km s$^{-1}$), (2) a small mass fraction of stars
($\sim$10%), and (3) a mean $B$-band surface brightness of the stellar
components fainter than 30 mag arcsec$^{-2}$. These results suggest that the
VIRGOHI21, which lies at a projected distance of $\sim$ 150 kpc from the
one-armed, HI-rich spiral galaxy M99 (NGC 4254), is tidal debris. We propose
that the comparison between the simulated and the observed velocity fields of
HI clouds allows us to better understand their nature and origin (e.g., whether
they are just tidal debris or ``dark galaxies'' that have HI gas only and are
embedded by dark matter halos). We also discuss the timescales for isolated HI
gas to be evaporated by thermal conduction of the hot intracluster gas.
