Shark

The Shark Egg is the Legendary-rarity egg of the Abyss Ocean, and the pet inside it is Shark. Abyss Ocean is zone 7 of 11 on the normal route; the wiki lists its entry requirement as ~2,5M speed, and the nests there are watched by the Moby.

Base income is $15K/s. Among the 8 pets tagged Abyss Ocean on the wiki that puts Shark at number 6 — El Maja tops the Abyss Ocean at $130M/s. Remember the game multiplies this base by the size roll and by the mutation, so two Sharks can earn very different amounts.

With a colour mutation and no size bonus, the same Shark would earn roughly: Silver ×1.2 ≈ $18K/s, Golden ×2.5 ≈ $37.5K/s, Rainbow ×3.5 ≈ $52.5K/s (multipliers from the wiki's Mutations page; arithmetic is ours). Add a Giant (×3) or Huge (×5) size roll on top and the numbers multiply again.

Hatching it pays a one-off reward of +90k Speed, $1.5m, and the egg takes 4m to grow in the pen. That reward is the reason low-income eggs are still worth stealing early: the speed bonus is what gets you into the next zone.

Legendaries are where a Golden or Rainbow roll starts changing your income bracket; guard the pen.