There are three quite similar exploits involving any beast that can be trapped for the Barn.
The first exploit is stealing someone else's beast without letting them cage it. Available only to classes with a taunt ability.
The second exploit is sharing the trap with someone else.
The third exploit is doubling your catch.
Exploit #1:
Available only to classes with a taunt ability: Warrior, Death Knight, Druid, Monk, Paladin, Hunter, perhaps a warlock with voidwalker.
It doesn't matter who tagged the mob first, as long as it is below 50% and walks over your trap.
1) Find someone who is already fighting a beast.
2) When the beast is about to reach 50% - go close, drop the trap and use the taunt ability.
3) Drag it through your trap.
4) Enjoy the stolen beast.
Exploit #2 (can be combined with #3):
1) Two or more players should stand exactly on the same spot facing the same direction.
2) All of them drop traps, they should be at an exact same location.
3) Get the beast to 50%, let it walk over the pile of traps.
4) Everyone should get a catch, unless someone has misplaced their trap.
Exploit #3 (can be combined with #2):
The timing depends on your DPS.
1) Attack the beast.
2) Drop the first trap at around 80%. Do not move, even an inch.
3) Damage the beast to 60%, don't get it lower than that. Stop DPS if necessary.
4) Drop the second trap as soon as the 15s cooldown is over.
5) Damage the beast past 50%, drag it through the pile of traps.
6) You should get two catches at the same time.
Enjoy!