I have seen people check in Macs in their original boxes. Wrapped into another box and a black bin bag.
However, be aware that the airline is not going to replace items of that value if lost or damaged. There's typically an upper limit on their insurance claim and it is typically lot lower than the value of your Mac Pro.
As a result of this I've heard some stories of checked in computers simply going amiss. They never arrived and are mysteriously unaccounted for. Someone just stole it.
And the insurance will pay you like $1,000 as compensation.
Another option might be to bring it along as carry-on hand luggage.
Seriously.
British Airways for example allows hand luggage up to 56cm x 45cm x 25cm (22" x 18" x 10"). Lufthansa on the other hand, and most airlines limits it to 55cm x 40cm x 20cm (22" x 14" x 9"). Check with your airline.
Seat Guru might help you compare maximum carry-on luggage sizes.
The Mac Pro's dimensions are: 51.1 cm x 47.5 cm x 20.6 cm (20.1" x 18.7" x 8.1").
Just a tad deeper than what British Airways allows, basically the handles are in the way. Yet they might still let you on the plane with it, as it's only the handles and they protrude merely 2.5cm (1 inch).
Otherwise you might have to find a way to remove the top or bottom handles. Filing them off or bending them inwards. Sounds harsh, but might be the safest, cheapest and quickest way to bring your Mac Pro along.
If you do bring your Mac Pro on the plane with you, wrap it in strong bubble wrap - but in a way that still allows you to open it up for inspections.
If carry-on luggage is not your thing, shipping it overseas with one of the moving companies might be your best bet.
I used
ExcessBaggage and was quite happy with them.
Shipping can take as long as 2-3 months, but from what I experienced (and heard from others) is usually well taken care of. My 19 boxes arrived in perfect condition because the moving company put them in their own wooden crate. (Much different story on my FedEx shipped boxes which arrived in an awful state - as if someone played football with them...)