Crustacia

The Crustacia Egg is the Legendary-rarity egg of the Titan Temple, and the pet inside it is Crustacia. Titan Temple is zone 11 of 11 on the normal route; the wiki lists its entry requirement as 7B speed, and the nests there are watched by the Gorilla King.

Base income is $130K/s. Among the 8 pets tagged Titan Temple on the wiki that puts Crustacia at number 6 — Nightflame tops the Titan Temple at 3B/s. Remember the game multiplies this base by the size roll and by the mutation, so two Crustacias can earn very different amounts.

With a colour mutation and no size bonus, the same Crustacia would earn roughly: Silver ×1.2 ≈ $156K/s, Golden ×2.5 ≈ $325K/s, Rainbow ×3.5 ≈ $455K/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 +2.7m Speed, $13m, and the egg takes 6m 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.