Lava Iguana

The Lava Iguana Egg is the Legendary-rarity egg of the Volcano, and the pet inside it is Lava Iguana. Volcano is zone 6 of 11 on the normal route; the wiki lists its entry requirement as ~700K speed, and the nests there are watched by the Cerberus.

Base income is $11K/s. Among the 8 pets tagged Volcano on the wiki that puts Lava Iguana at number 5 — Lava Dragon tops the Volcano at $100M/s. Remember the game multiplies this base by the size roll and by the mutation, so two Lava Iguanas can earn very different amounts.

With a colour mutation and no size bonus, the same Lava Iguana would earn roughly: Silver ×1.2 ≈ $13.20K/s, Golden ×2.5 ≈ $27.5K/s, Rainbow ×3.5 ≈ $38.5K/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 +36k Speed, $1.1m, 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.