And the mystery is solved. . . from the United web site:Both Continental and American. Return flight to Belize from the UK was going to cost me an extra US$400.
United accepts one dive bag containing diving or scuba equipment as part of a customer's standard checked baggage allowance. A dive bag containing scuba equipment that exceeds the standard checked baggage allowance will be charged oversize or overweight baggage charges.
Dive bags weighing more than 50 pounds (23 kg) that contain other items in addition to or in place of appropriate dive equipment will be subject to the applicable overweight checked baggage service charge.
Dive bags measuring greater than 62 linear inches (158 cm) that contain other items in addition to or in place of dive equipment will be subject to the applicable oversize charge.
First or second checked bag service charges may apply.
An empty dive tank or up to three rebreather tanks will not be included in determining the baggage allowance and will be subject to a service charge of $150 USD/CAD (each way) for travel between the U.S., Canada, Puerto Rico and the U.S. Virgin Islands. A service charge of $200 USD/CAD (each way) applies for all other travel. The empty dive/rebreather tank must have the regulator valve completely disconnected from the tank. The tank must not be sealed (i.e., the tank has an open end). The tank must have an opening to allow for a visual inspection by a TSA Security Screener.
This seems bit like an intelligence test. Pack your pony in a dive gear bag and it will cost $200 each way. Pack your pony in your suitcase and it rides for free (assuming you stay under the weight limit).
Gee. . . that is a tough one. . .