Iberia
From https://www.iberiaexpress.com/en/general-info/passenger-information/before-you-go/baggage/checked-baggage/special-baggage
Please remember that safety considerations determine the ability for the aircraft hold to carry items of special baggage. Therefore, in some cases it may not be possible to check in these items.
Space reservation: In order for special baggage to be carried on the flight, it must be accepted beforehand and a space reserved at least 24 hours prior to check-in.
Checking-in at the airport: The passenger must arrive at the airport with sufficient time for their luggage to be checked in, transported to the plane and loaded in the hold.
Loading of equipment: Due to their size or shape, special baggage normally cannot be transported on airport conveyor belts. The use of an alternate system to make transporting such items from the check-in area to the aircraft easier is at the discretion of company staff. The passenger will bear any costs that may arise from transporting the equipment.
Luggage that must be checked-in:
A charge of 45 euros* will be applied, irrespective of luggage allowance, to sporting goods and musical equipment.Payment for these items must be made at the check-in desk.
*Except firearms and at London Heathrow Airport)
**Different taxes, charges and fees may apply, in accordance with the relevant legislation in each country.
Exception: For Iberia Singular, Iberia Plus Infinite, Platinum, Gold and Silver cardholders, these items shall always be considered as part of the luggage allowance. This advantage is not extended to oneworld holders.
Bicycle
A non-motorized single-saddle bicycle. Must be checked in with pedals removed, tyres deflated and must be perfectly packaged.
Price: 45 Euro

From http://www.iberia.com/web/program.do?menuId=EQUIEE&isPopup=true&AAA=true
Bicycles
Please fold down the handlebars, remove the pedals, take the air out of the wheels and pack it in one of the special boxes we will provide you with.
Carriage requirements
Single-seat and no motor.
It must be checked-in folded, with the handlebars against the sides, the pedals removed, the wheels with the air taken out and correctly packed.
Iberia has a packing container measuring 131+72+21 cm in many airports. The price of this container is 20 Euros per unit. This container will be issued at the customer's request or at the requirement of the agents in cases of irregularities (except, e.g., in Berlin and Dusseldorf airports). If the bicycle being checked in is bigger than the container provided by IBERIA, the passenger will be responsible for finding another one that is the right size.
Iberia reserves the right to reject bulky sporting equipment and to pass it to Iberia Cargo with conditions and tariffs that are different from those indicated above.
Allowance
For long-haul flights (America, Asia and South Africa), it is considered an item that can be carried within the passenger's allowance. If you add other items, exceeding the permitted number of free items, it will be considered excess baggage.
For short-haul and medium-haul flights, a single fee of 45 Euro applies, regardless of the luggage allowance.
The maximum weight allowed shall be 23 kg with an overweight fee for weights of up to 32 kg. The fee must be paid at the airport.