I'm mapping a legacy DB which has an implicit relationship
between two tables. This can of course be handled at the application
level but I would love to create a hibernate mapping if possible.

Its a many-to-one or zero association. Let's say from M to OZ.
OZs primary key is a value, say REF in M that will never be null.
The important point is that there isn't an OZ for every REF in M.

I have been able to map it fine using many-to-one except that when
OZ doesn't exist for a REF, hibernate tries to hydrate the OZ anyway
and I get a NullPointerException (as expected).

Is there anyway to map this that hibernate won't freak out?

Use a UserType.

If I created a type, what would the mapping I create
be?

I've been looking hard at the UserType class, CompositeUserType
class and tests that implement them but I just don't see how it solves
my problem. It looks like I could create a collection of some type
in M (from my example above) using a custom type but I don't see
how to map it or create it so that M doesn't end up with a collection
of all OZs. First of all, there will only be one OZ for each M if there
is one at all. A collection is impractical. In our case there are roughly
a half million OZs and that number will probably double or triple in
a year or so, and keep growing after that. I dont' see how to create a UserType and map it such that that there is only one of my custom type (representing an OZ) based on the OZ key in M.