The group – People for the Ethical Treatment of Animals – sent teenager Logan Ward pictures of scantily-clad 'Lettuce Ladies' to try to persuade him to save the cow.
But their initially light-hearted protest turned nasty when PETA supporters bombarded the boy with messages describing him as 'disgusting' and 'heartless' for trying to sell the cow.
The strange saga began because Loga, from Sidell, Louisiana, couldn't afford to go on a school trip to Britain and Ireland.
So his dad, Scott, convinced a friend to donate a cow, then to sell raffle tickets to win the animal at $10 a time in the hope of raising $8,000.
PETA got wind of the raffle and sent Logan an email offering the services of 'Lettuce Ladies' - models who wear nothing but skimpy lettuce leaf bikinis and wrestle in tofu – along with an email from employee Whitney Calk.
She wrote: "Call me crazy, but it just seems a little strange to me that you feel that someone needs to die in order for you to go on a school trip. Know what I mean?
"Would you consider allowing me to work with you to move the cow to a reputable sanctuary where he or she can live out the rest of his or her days freely (as nature intended)?
"In return, I'd like to work with you directly on your fundraising efforts by helping you raise money in a way that doesn't cost someone their life. One idea I'd like to suggest would be a veggie dog sale in your community hosted by PETA's bikini-clad Lettuce Ladies."
PETA uses its Lettuce Ladies to convince people to give up meat and begin eating an all-vegan diet. But Logan's dad was not impressed.
"When we received the email we were absolutely incensed... and then obviously the links to the so-call 'Lettuce Ladies' - that was wildly inappropriate for a 14-year-old boy," he told Fox News.
PETA later apologised, saying the group sent the wrong form message to Logan, but Scott said the group's actions sparked a wave of cyber bullying of his son.
Cathy Matthews wrote on Facebook: "You're disgusting and heartless. Here's reflecting all your vileness back to you. Hope whoever wins that poor animals flesh chokes on it."
Another, Claire Jenkins, wrote: "Not welcome in UK thank you!! This is sick!"
Another Briton, Vicky Fraser, said: "A life is taken for a scrounging brat to go on holiday? Great morals to drag your children up with."
However, the Ward family has also been flooded with donations and support since the story went public.