I hope I'm not too late. :)
Mega Nihilego (UB-01-Mega: Jellinator)
Ability: Beast Boost
Typing: Rock / Poison
HP: 109
Attack: 53 -> 23 (-30)
Defense: 47 -> 117 (+70)
Special Attack: 127 -> 157 (+30)
Special Defense: 131 -> 141 (+10)
Speed: 103 -> 123 (+20)
New Moves: Calm Mind, Nasty Plot
Description:
Going straight up to AGg.
Mega Nihilego, is big, bad and made of jelly. It truly lives up to the nickname I gave it when it was first announced: the Jellinator.
If you want to know what it looks like, picture what Lusamine looked like when she combined with Nihilego. It looks like that.
Mega Nihilego has great 109 / 117 / 141 bulk, meaning that even with its questionable defensive typing, it can take a hit or two. Base 157 Special Attack breaks massive holes in everything. Two new setup moves in Calm Mind and Nasty Plot gives Mega Nihilego many options and makes it unpredictable.
Nobody can beat the Jellinator.
Mega Xurkitree (UB-03-Mega: Lightinator)
Ability: Beast Boost
Typing: Electric
HP: 83
Attack: 89 -> 59 (-30)
Defense: 71 -> 100 (+29)
Special Attack: 173 -> 217 (+44)
Special Defense: 71 -> 100 (+29)
Speed: 83 -> 101 (+18)
New Move: Agility
This one might not as be as OP as lord and savior Jellinator, but it definitely has something going for it.
Mega Xurkitree is covered in white fluffiness, which somehow is even better with electricity. The wires being covered up gives Xurkitree more defenses.
Base 217 special attack is ridiculous, and 101 is a great speed tier. Good mixed bulk means that Mega Xurkitree will really sent the opposing team before going down.
Mega Guzzlord (UB-05-Mega: Terminator)
Ability: Beast Boost
Typing: Dark / Dragon
HP: 223
Attack: 101 -> 121 (+20)
Defense: 53 -> 108 (+55)
Special Attack: 97
Special Defense: 53 -> 108 (+55)
Speed: 43 -> 13 (-30)
New Move: Swords Dance, Soft-Boiled (it eats eggs, and then cracks them?)
Mega Guzzlord has an even bigger mouth than before, and the smaller arms at the top of its body become long enough to actually be useful. Its wings also become fully grown.
Guzzlord should be never to BULKlord, as it finally has the bulk it needs to support that massive base 223 HP stat. Attack is great too, and 223/108/108 bulk is fantastic. Soft-Boiled cos why the hell not (the only slightly realistic recovery move for this guy).