Courtesy of Sugar Cookie.
The South American rodents were among 20 who were flown from the Peruvian capital of Lima to London, via Madrid.
Staff at Heathrow Airport were immediately concerned when the Iberia plane landed on October 28 last year.
The capybaras were found in cramped cages without any water or sufficient ventilation.
Two of them were found dead and a further 10 died in the reception area of the London airport, Uxbridge Magistrates Court heard today.
The court was told the creatures started their journey in Lima, with Iberia's Chilean partner Latam Airlines, before being flown by Iberia from Madrid to Beijing via London Heathrow.
Prosecutor Jaysen Sharp said: 'The crates did not have dispenser bars and insufficient ventilation.
'The animals were inspected and two were found to be deceased on arrival. The remaining 18 animals appeared to be in bad shape.'
He said one of the rodents died before it could be taken out of its crate.
'Another one collapsed, appeared to be drooling and showing cerebral dysfunction.'
A vet examined the suffering Capybara and it had to be put down.
Another 10 animals died in the reception centre and the surviving eight were eventually put on a flight to Beijing.
A post-mortem examination was performed to determine the cause of their deaths.
'The cause of death was due to the lack of water provided in the transport boxes, and the very young age of some of the animals which caused unnecessary suffering,' added the prosecutor.
Edward Smith, defending the airline, said: 'It is absolutely mortified by this incident.
'It has no previous convictions of anything like this. It is incredibly sorry and takes this matter most seriously.
'As you can appreciate Iberia is the national carrier for Spain and takes this matter incredibly seriously,' he added.
'Transport of live animals in this manner will not take place without the prior advance warning.'
Following the incident, the airline sent a bulletin to staff members in order to avoid a possible repetition of the tragic events.
'The sales and operation team in Lima shall be informed of the correct way of transporting live animals,' said Mr Smith.
Mr Smith said the first part of the journey between Lima and Madrid was taken on by Iberia's partner Chilean Latam Airlines.
'There is an ongoing investigation with our client and Latam.
Iberia admitted causing unnecessary suffering to 20 capybaras by failing to supply enough water during transportation.
The Spanish airline now faces sentence at Ealing Magistrates' Court on Friday.
Two of the capybaras were found dead and a further 10 died in the reception area at London Heathrow airport, Uxbridge Magistrates Court heard today.