Last week when I went to raid with my guild, I summoned my usual spirit beast (who I use exclusively for raiding/m+) and noticed he had this strange golden glow, complete with sound effects:
I thought at first it had come from a buff, but when I click on him he has no unusual buffs/debuffs. The effect persists through being dismissed and re-summoned, and though I'm not 100% sure, I think it may have also persisted through my pet's death in a m+ run too. I haven't tested on arenas yet as I'm a bit reluctant to risk it since I'm quite enjoying the unique effect.
However, I have no idea where I picked it up from. I have a suspicion it may have something to do with codex of chromie, but I had a completely different pet out when I was doing it who is unaffected, as is my secondary stable slot pet (who I use for everything). If anyone can confirm where this effect is from, and even better yet figure out how my pet got stuck with it, I'd love to know!