Sounds like Red Rising by Pierce Brown. Published 2013 which would fit.
Goodreads summary:
Darrow is a Red, a member of the lowest caste in the color-coded society of the future. Like his fellow Reds, he works all day, believing that he and his people are making the surface of Mars livable for future generations.
Yet he spends his life willingly, knowing that his blood and sweat will one day result in a better world for his children.
But Darrow and his kind have been betrayed. Soon he discovers that humanity already reached the surface generations ago. Vast cities and sprawling parks spread across the planet. Darrow—and Reds like him—are nothing more than slaves to a decadent ruling class.
Inspired by a longing for justice, and driven by the memory of lost love, Darrow sacrifices everything to infiltrate the legendary Institute, a proving ground for the dominant Gold caste, where the next generation of humanity's overlords struggle for power. He will be forced to compete for his life and the very future of civilization against the best and most brutal of Society's ruling class. There, he will stop at nothing to bring down his enemies... even if it means he has to become one of them to do so.
The referenced 'institute' is entered by competing in what are effectively war games on Mars, where the young adults are in houses, each with an assigned Proctor. Many of those who start the competition die in it.
One of the others - Sevro - leads a squad referred to as 'Howlers', who did indeed hide in animal carcasses at one point - specifically horses:
Someone screams as they see Sevro and his Howlers cutting their way out of the stitched-up bellies of the dead and bloated horses that litter the mud up to the gate. Like demons being born, they slither from swollen guts and parted stomachs. A half-score of House Diana’s best soldiers exit with them. Tactus and his spiked hair burst from the belly of a pale mare. He runs with Weed and Thistle and Clown. All within fifty meters of the ponderously slow gates.
