This is what I don’t understand. The Red Soul Shard and The Butcher Pet are rewards for the same Achievement. If you received one, you should have received both, as well as the Achievement: An Eerie Red Glow Blurs Your Vision.
Check your Achievements again. It will be located in:
Achievements → Career → General → Special Events
Also check your Collections again for the Pet. (I know you’ve already done this. But, just to be sure).
If all else fails, try Maskraider’s suggestion. If you already started the Event with a Level 1 Character and had 0 XP when you entered the Portal to Tristram, you should be able to use that same Character again. (Despite having Leveled, have Paragon Points and gear) You can even set the game on Normal Difficulty. Just run all 16 Levels in one gaming session and don’t die.
If you still don’t get it, you can create a new, temporary, Character and try again. I have a detailed guide for this Event here, which might help you out, if you think you need it.
Best of luck!