From the DOT website:
Under what circumstances may airlines deny transport to a service dog?- Airlines are permitted to deny transport to a service dog if it:
- Violates safety requirements - e.g., too large or heavy to be accommodated in the cabin;
- Poses a direct threat to the health or safety of others;
- Causes a significant disruption in the cabin or at airport gate areas; or
- Violates health requirements - e.g., prohibited from entering a U.S. territory or foreign country.
And there is the affidavit from the owner stating the animal won’t relive itself and/or won’t take a dump in the aft galley.
Animal Relief Affidavit