  We present a Spitzer/IRS low-resolution mid-infrared spectrum (5-14 micron)
of the luminous transient discovered in the nearby galaxy NGC 300 in May 2008.
The spectrum, obtained three months after discovery, shows that the transient
is very luminous in the mid-IR. Furthermore, the spectrum shows strong, broad
emission features at 8 and 12 micron that are observed in Galactic carbon-rich
proto-planetary nebulae. Combining these data with published optical and
near-IR photometry obtained at the same epoch, we find that the mid-IR excess
traced by the Spitzer spectrum accounts for ~20% of the total energy output.
This component can be well explained by emission from ~3x10^-4 Msun of
pre-existing progenitor dust at temperature ~400 K. The spectral energy
distribution of the transient also shows a near-IR excess that can be explained
by emission from newly-formed dust in the ejecta. Alternatively, both the
near-IR and mid-IR excesses can together be explained by a single pre-existing
geometrically thick dust shell. In light of the new observations obtained with
Spitzer, we revisit the analysis of the optical spectra and kinematics, which
were compared to the massive yellow-hypergiant IRC+10420 in previous studies.
We show that proto-planetary nebulae share many properties with the NGC 300
transient and SN 2008S. We conclude that even though the explosion of a massive
star (> 10 Msun) cannot be ruled out, an explosive event on a massive (6-10
Msun) carbon-rich AGB/super-AGB or post-AGB star is consistent with all
observations of the transients and their progenitors presented thus far.
