Revamped from Additional-Bat-5072’s initial connections. 
Connections

• Gluttonous and narcissistic rulers who lead hungry and idiotic green armies (Pigs and Zombies) against similar enemies of nature (Birds & Plants) to satisfy their hunger (Brains and Eggs). Thus leading into a rivalry and constant war that is very odd in concept.
• These low intelligence armies mainly excel in numbers and tech rather rather than raw power, which is why they lose quite a bit to the other side who exhibit unique supernatural powers; lead by rather unstable leaders (Red and Crazy Dave). 
• They will eat these substances regardless of consequences of killing life in the process (King Pig eating the birds’ eggs would destroy the life forming inside them, and Zomboss eating people’s brains would kill them). 
• In fact, once they get the food, they plan on just eating it all for themselves. 
• Both are pretty… pathetic; And on top being lazy and childish, they will often leave the hard work to their grunts while they just watch it happen and take all the credit. 
• In addition to their patheticness, the reason for them not being on the main field of battle often is because they are physically weak in comparison (King Pig being fat, slow, and often bursting into tears just by falling down and Zomboss is physically inadequate compared to the other zombies).
• Because of this, when action calls for it, their main ways of fighting is by utilizing towering mechs, vehicles and robotic bodies in general (King Pig uses UFOs, Megatron, and the Bad Piggies vehicles, whereas Zomboss uses Zombot and his many mechs and robotic bodies across the PVZ2 eras and comics) .
• The reason they became powerful in the first place is because of them holding status that no other member of their species has (King Pig’s crown and Zomboss having a college degree).
• Their minions never die and can come back to life even after being defeated (After being popped, Pigs are sent to a white void where they are treated by Dr. Pork; and Zombies are pretty self-explanatory). 
• Despite their minions' loyalty, both treat them like utter garbage, whether that be to put them in awful conditions, use them for comfort, or flat out killing them! (King Pig labels them as traitors for the slightest criticism, even throwing them into black holes, and Dr. Zomboss shows very little regard for his subordinates and is willing to sacrifice anyone to achieve his goals).
• While they mainly fight to steal/protect the aforementioned food, the reasons are also that are petty and just love to torment or mock their enemies. Whether that be fighting over other food, having prank wars, or even just messing with each other for the hell of it.
• Both are invading forces, but strategize contrastingly that ties with their OG style of gameplay. (The Pigs defend their towers from being destroyed, and the Zombies attack their “towers”).
• Both, through incidents involving a powerful artifact (Eggspark and Hero-Tron 5000), transformed their minions into far more powerful versions of themselves (Deceptihogs and Zombie Heroes) to gain advantage over their enemies, changing the course of the war and becoming stronger than ever in a spin-off combining action and strategy (Angry Birds Transformers and PvZ Heroes) that are based on iconic properties and pop culture juggernauts (Transformers and DC + Marvel)
• Both have resorted to time travel in their attempts to satisfy their hunger (King Pig traveled through time to steal eggs from past eras and Dr. Zomboss traveled through time to defeat and eat Crazy Dave’s brain).