Ankylosaurus in Steal an Egg

Ankylosaurus (Mythic rarity) belongs to the Prehistoric zone. Prehistoric is zone 8 of 12 on the wiki's Biomes list; the wiki lists its entry requirement as 18M speed, and the nests there are watched by the T-Rex. You get it by carrying the Ankylosaurus Egg back to your pen.

At $120K/s base income, Ankylosaurus sits 6th out of 8 in its zone — Mosasaurus tops the Prehistoric at $180M/s. The listed figure assumes no mutation and a normal weight.

With a colour mutation and no size bonus, the same Ankylosaurus would earn roughly: Silver ×1.2 ≈ $144K/s, Golden ×2.5 ≈ $300K/s, Rainbow ×3.5 ≈ $420K/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 +330k Speed, $12m, 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.

Mythics are the eggs other players come to steal. Bear traps around the pen are the cheap insurance (max 3, 7-second hold).