Missing Mega Evolutions in Shuffle

Released thus far:

1. Mega Charizard Y
2. Mega Heracross
3. Mega Sharpedo
4. Mega Blaziken
5. Mega Absol
6. Mega Manectric
7. Mega Garchomp
8. Mega Latias
9. Mega Latios
10. Mega Gardevoir
11. Mega Steelix
12. Mega Medicham
13. Mega Swampert
14. Mega Charizard X
15. Mega Mewtwo X
16. Mega Abomasnow
17. Mega Metagross

Mega Evolutions of known Pokémon:

1. Mega Charizard X
2. Mega Pidgeot
3. Mega Scizor
4. Mega Mewtwo X
5. Mega Pinsir
6. Primal Groudon
7. Primal Kyogre
8. Mega Rayquaza
9. Mega Tyranitar
10. Mega Sceptile
11. Mega Swampert
12. Mega Camerupt
13. Mega Salamence
14. Mega Gyarados
15. Mega Diancie
16. Mega Gallade
17. Mega Beedrill
18. Mega Alakazam
19. Mega Aggron
20. Mega Houndoom
21. Mega Abomasnow
22. Mega Steelix
23. Mega Medicham

Unknown Pokémon with Mega Evolutions:

1. Mega Houndoom
2. Mega Gardevoir
3. Mega Latios

Edit(07.09.2015): Current pre-paid cards showcase Mega Sceptile and Mega Swampert, but they have not been released.
Edit(28.09.2015): Removed Mega Latias and Mega Charizard Y. Updated the Unknown Pokémon with newly released stuff.
Edit(05.10.2015): Removed Mega Absol.
Edit(02.11.2015): Removed Mega Gardevoir.
Edit(01.12.2015): Removed Mega Steelix, Mega Medicham and Mega Latios. Moved Gallade to known Pokémon.
Edit(28.12.2015): Moved Camerupt to known Pokémon.
Edit(05.12.2015): Removed Mega Swampert.
Edit(20.01.2015): Moved Aggron and Abomasnow to known Pokémon.
Edit(09.02.2016): Removed Mega Abomasnow.
Edit(16.02.2016): Removed Mega Mewtwo X. Added Beedrill and Alakazam to known Pokémon.
Edit(23.02.2016): Removed Mega Charizard X. Added Gyarados to known Pokémon.
Edit(15.03.2016): Added Houndoom to known Pokémon.
Edit(05.04.2016): Removed Mega Sceptile.
Edit(26.04.2016): Removed Mega Scizor, Mega Diancie and Mega Rayquaza.
Edit(28.06.2016): Removed Mega Gyarados.
Edit(02.08.2016): Removed Mega Steelix.
Edit(13.09.2016): Removed Mega Alakazam.
Edit (28.09.2016): Removed Mega Beedrill.
Edit (11.10.2016): Removed Mega Pidgeot.
Edit (01.11.2016): Removed Mega Metagross.
Edit(22.11.2016): Removed Mega Camerupt and Tyranitar.
Edit (21.12.2016): Removed Mega Salamence.
Edit(17.01.2016): Removed Mega Houndoom.
Edit(14.02.2016): Removed Mega Aggron.
Edit(13.06.2016): Removed Mega Gallade.
Edit(13.02.2018): Removed Primals. And condensed the list.
Edit(08.05.2018): Removed Mega Pinsir. All Megas are now available.