Here's some choices I personally think accompany Charizard-Y very well:
Venusaur
Venusaur @ Black Sludge
Ability: Chlorophyll
EVs: 252 SpA / 252 Spe / 4 HP
Modest Nature
- Protect
- Sleep Powder
- Sludge Bomb
- Giga Drain
Venusaur + MegaZard Y is, I daresay, the most popular and powerful Sun lead presently. Chlorophyll gets boosted in the sun, and boosts Venusaur's speed sky high, it even outspeeds common Choice Scarf Pokemon like Landorus-T and Terrakion. This allows it to either aim for a kill, or a quick sleep status on the opponent. A well aimed Sleep Powder can doom your opponent. Sludge Bomb and Giga Drain are for STAB, also hitting Fairy and Water types like Rotom-W. Spread can be tweaked for more specific stat ranges.
Landorus-T
Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 44 Def / 212 Spe
Adamant Nature
- U-turn
- Superpower
- Rock Slide
- Earthquake
Landorus is one of the most common Pokemon in this metagame, and can fit into almost all teams relatively well. It takes out stuff that wants to take out MegaZard, such as Rock and Electric types not named Rotom-W (Zard's Solar Beam does hefty damage to it so that shouldn't be a problem). Examples include Raikou, Tyranitar, Terrakion and Thundurus. It also provides much appreciated Intimidate support, helping Zard take physical hits on it's way to rampage.
Mamoswine
Mamoswine @ Life Orb/Choice Scarf
Ability: Thick Fat
EVs: 196 HP / 252 Atk / 60 Spe
Adamant Nature
- Ice Shard
- Earthquake
- Icicle Crash
- Rock Slide / Protect
Charizard really appreciates help from fast Pokemon, because it's speed is relatively middling i.e it's not exactly slow, but it fails to outspeed important stuff. Things like Thundurus, Lando-T, Terrakion just completely throw it off it's game. Mamoswine is a good Pokemon to consider as a teammate as it can not only take out all threats Charizard would rather avoid, but also spam Earthquake with no fear of damaging it's teammate. Choice Scarf is advised here, but Life Orb is also good if you deem the extra power necessary.
There are plenty of other Pokemon that also function very well under MegaZard Y's wing, but these are the major ones. Hope I helped!
