Quick answer
Weight drives both the value of an animal and the income it generates. Heavier animals come from bigger eggs, so the reported strategy is to break every egg in a zone and steal only the highest-earning animal you can escape with. Egg colors are cosmetic; size and rarity are what matter.
Why weight is the stat that matters
Every animal in your pen earns Cash over time, and the amount it earns is mainly determined by its weight. Two animals of the same species can therefore be worth very different amounts depending on how heavy they are. That is why the size of the egg you break is a better guide than its color: bigger eggs are reported to hold heavier animals.
The stealing strategy guides agree on
Because a zone's eggs share one rarity tier and hatch on RNG, the efficient loop is to break every egg in the zone, compare what hatches, and carry home only the highest-earning animal you can escape with. Weight transfers through the whole chain: bigger egg, heavier animal, more Cash per second in your pen.
Duplicates are not dead weight
Three animals of the same species, of any weight, can be merged into an egg that hatches a heavier variant of that species. Submitting heavier animals to the merging machine raises the weight of the egg you get back, so a duplicate is better understood as an upgrade material than as clutter.
What is not verified
No source this site trusts publishes per-animal income values, hatch odds or a verified best-animal ranking for this game. Any per-second income table or tier ranking would therefore be invented, so this page sticks to the weight rules that the available guides do support.
Frequently Asked Questions
Why are heavier animals better?
Weight drives both the value of an animal and the Cash it generates, and heavier animals hatch from bigger eggs. Duplicates are not dead weight either: merge three of the same species and the machine returns an egg with a heavier variant of that species.
What is the best animal in the game?
There is no verified answer yet. Guides agree that you should steal the highest-earning animal you can escape with from each zone, but no source this site trusts publishes per-animal income values, so this site does not rank animals it cannot check. Secret-rarity eggs in the Galaxy Zone are reported to hold the rarest animals.
Related resources
Pets
How Break and Steal an Egg animals work: obtaining them from eggs, weight-driven income, pen capacity, merging and index rewards.
Eggs
Every reported Break and Steal an Egg egg by zone, with rarity tiers, guardian difficulty and the break-steal-hatch loop.
Tier List
A source-backed ranking of zones, treadmill and trail upgrades, and egg targets with visible methodology.