Federal animal laws mostly deal with animals used in research, the transport of animals and livestock across state lines, and the prevention of the transfer of disease from foreign countries into the United States.
The Federal Law Website has a list of federal laws of all types, including ones related to animals. Sample laws include: Animal Cancer Research Act, Animal Disease Control Cooperation Act of 1947, Animal Drug Amendments of 1968, Animal Enterprise Protection Act of 1992, Animal Industry, Animal Medicinal Drug Use Clarification Act of 1994, Animal Quarantine Acts, Animal Virus, Serum, Toxin, Antitoxin Act, Animal Welfare Act, Animal Welfare Act of 1970, and the Animal Welfare Act Amendments of 1976.
The US Code Website is a general federal law website.
The House of Representative Website is a good resource for pending laws.
The Office of the Law Revision Counsel prepares and publishes the United States Code, which is a consolidation and codification by subject matter of the general and permanent laws of the United States. Another source for federal law listings is the National Archives and Records Administration site.