The Deathstalker Rexxar’s Hero Power called Build-A-Beast that allows you to merge two beast minions that are already on the battlefield to create a more powerful minion known as a Zombeast. It all works fine and its currently very popular along with the new hero cards that have been added to Hearthstone.
However, for some reason, beast minions from the new Kobolds & Catacombs Expansion are not affected by Deathstalker Rexxar’s Hero Powers leaving many players confused. However, some have speculated that some beasts are locked as Deathstalker Rexxar and some of the other hero cards will get their own unique minions that could potentially be really powerful. We have to wait and see as I personally doubt its a mistake from Blizzard.