Are bloodhounds the only animal whose evidence is admissible in US courts? Where would I find the "law" that states this?
The law might vary by state (and you did not indicate what state you are in). But, no, a "bloodhound" is not the only animal whose alert can be utilized as probable cause for a search. Generally, most any dog can be used provided the handler or agency can show the court that it has been properly trained and has a track record of reliability.
You can probably contact a local or state K-9 handler's association for more details ont he laws in your state.