Tiger Knights

So one day, me and my dad were playing Spartan ops EP:6 CH:4 (search and destroy) on legendary difficulty and when it was close to the end (where you need to press a button in the orange area) there were these 3 Knights that stood out from the rest. They looked like regular Knights, except that they were covered head-to-ankle in orange-glowing stripes. Two of them wielded incinerator cannons while one sniped with a binary. me being me, I ran up to one of the “Tiger Knights” and punched it. It didn’t do a lot since it was on legendary difficulty. But they were much stronger than a commander. And they were tag-teaming against me and my dad! And they only appear on legendary for some reason. Can anyone explain these mysterious Super Knights and why they only spawn on legendary?