This is an interesting question, drosen; thanks for posing it.
drosen0000@wahoo.com wrote:

Since none of the other vertebrate classes have a male copulatory organ
homologous to the mammalian penis it is going to be difficult to find a
homologous structure. I did a Google search on "baculum homology" and
found some interesting papers you might be interested in looking at.
Here's a snippet from
Acta Chiropterologica, 5(1): 117-123, 2003
The ontogeny of the baculum in Nyctalus noctula and
Vespertilio murinus (Chiroptera: Vespertilionidae) by
DMITRY G. SMIRNOV...

I don't have the references anymore on this. However, I remember
quite clearly about the parietal and dental bones.
The parietal bones (a pair of bones at the very top of the skull)
are derived from the ectoderm, not the mesoderm. The parietal bones
have always been called bones to my knowledge. There seems to have been
a post Cambrian event that formed the parietal bones. Some fossilized
fish are lacking analogs to the parietals, although later fossilized
fish have them. In any case, embryological studies have indicated that
the parietals come from ectoderm. Most other skull bones come from
mesoderm. The parietals are sometimes called the "second skull."
The hard parts of teeth are derived from ectodermal tissue.Sharks
have placoid scales all over that are homologous to teeth. The enamel
and dentin of teeth are derived from the ectoderm. The pulp of teeth
contain blood vessels and nerves that are derived from the mesoderm.
The enamel and the cells that generate it are ectodermal.
You are quite possibly right concerning armadillo armour and
bacula. They may be dermal bones without coming from the ectoderm. I
made an incorrect projection based on their location. Structures ...

Read:
http://dev.biologists.org/cgi/reprint/117/2/409.pdf
These guys claim that the top of the skull is derived from neural
crest tissue, which is ectodermal. I Googled the topic. Some say the
parietals are ectodermal, some say that it is mesodermal. I think the
ectodermal origin may be still controversial. I am not so sure anymore.

The baculum (i.e., os penis) is a bone found in most mammal
species other than human beings and pygmy chimpanzies (i.e., bonobos).
Is there a feature similar to the baculum in structure (i.e.,
homologous) in nonmammalian species?

I know there are features that are close to the same location as
the baculum in other species (thank you, Hoyt, for explaining these to
me), showing that these features may be analogous in function. However,
I am interested strictly in homologies. The current use of the baculum
in mammals is unknown. The large variation in the baculum in mammal
species shows that the current use in mammals is probably nonessential.
However, there may be a homologous structure outside of the mammal
class that is essential to that animal.

I stated the question entirely wrong before, based on
missunderstanding of the baculum. Therefore, I am reposting the
question.

I think de novo features are uncommon relative to modified
features. The reason is that a de novo feature would most probably
arrive by saltation. A saltation is a sudden abrupt mutation, evident
in one or a small number of generations. Saltations are less likely to
improve fitness than an accumulation of small variations. My interest
in the baculum is that it may be one of those rare instances of
saltation. But maybe saltations are more common.

drosen0000@wahoo.com wrote:

I think you're confusing dermal bone with ectoderm, an embryonic germ
layer. Dermal bones arise from the dermis, which is mesodermal in
origin.

In the bat baculum paper I cited they indicated that the baculum arose ...