Salamander in Steal an Egg

Salamander (Legendary rarity) belongs to the Cherry Blossom zone. Cherry Blossom is zone 10 of 12 on the wiki's Biomes list; the wiki lists its entry requirement as 2.5B speed, and the nests there are watched by the Oni Tiger. You get it by carrying the Salamander Egg back to your pen.

At $74K/s base income, Salamander sits 7th out of 8 in its zone — Kitsune tops the Cherry Blossom at $1.8B/s. The listed figure assumes no mutation and a normal weight.

With a colour mutation and no size bonus, the same Salamander would earn roughly: Silver ×1.2 ≈ $88.80K/s, Golden ×2.5 ≈ $185K/s, Rainbow ×3.5 ≈ $259K/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 +3m Speed, $7.4m, and the egg takes 5m 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.