During Marvel's summer Empyre event, the long-warring Kree and Skrull races united under one banner, joining the heroes of Earth to fight against the plant-based forces of the Cotati. In the tie-in issues of Fantastic Four, the children of the Reed and Sue Richards - Franklin and Valeria - are tasked with protecting two very important alien children, one Kree Boy and a Skrull girl. When the alien children proved challenging, Franklin Richards makes a reference to the Fantastic Four's first altercation with the Skrull race, which was in fact pretty dark.
Together with Spider-Man and Wolverine, the Fantastic Four did all they could to fight the Cotati and protect the children, but the Kree and Skrull youngsters were hard to handle, as they both wanted to kill the other. They had long been pitted against one another for the entertainment of the Profiteer and the patrons at her casino before they were rescued from by the Fantastic Four. Seeing as how they were the last symbols and representations of the Kree/Skrull War, the Cotati wizards sought to amplify their feelings of animosity across the stars in order to destroy the alliance between the two races.
In Fantastic Four #21, from writer Dan Slott and artists Paco Medina and Sean Izaakse, Jo-Venn (the Kree) and N'Kalla (the Skrull) can't help but fight one another. It's what they were bred for and have done since they were born. However, in order to keep them safe, Valeria and Franklin Richards have to get them to call a truce. While Valeria appeals to Jo-Venn's honor as a Kree to get him to stop fighting, Franklin makes a threat to N'Kalla while she's shape-shifted into the form of a tiger, and the threat is pretty dark.
The moment that Franklin is referencing comes from the first appearance of Skrulls in the Marvel Universe and their twisted fate. In Fantastic Four #2, from Stan Lee and Jack Kirby, Reed Richards does indeed force the first Skrulls they ever fought to turn themselves into cows, and then hypnotized them into thinking they were really bovine for the rest of their lives. It's quite the cruel punishment, and one that the young N'Kalla would certainly be frightened by. Sure enough, she starts listening to the Richards children, not wanting to be turned into a cow herself.
As the rest of Empyre continues, Jo-Venn and N'Kalla end up reconciling and becoming allies, bolstering the alliance of the Kree and Skrull instead of breaking it apart. However, it is somewhat tarnished by the fact that Franklin resorted to such a strong threat at its onset. Sure it was meant to be a humorous reference for Fantastic Four fans, but for N'Kalla is was probably traumatizing to think about being turned into a dumb animal for the rest of her life. While Franklin got his desired result, he certainly didn't pull any punches in doing so. In any case, things seem fine now that Empyre has concluded and the two children were adopted by the Thing.