Mammoth

The Mammoth Egg is the Mythic-rarity egg of the Snow, and the pet inside it is Mammoth. Snow is zone 5 of 11 on the normal route; the wiki lists its entry requirement as ~170K speed, and the nests there are watched by the Yeti.

Base income is $42K/s. Among the 8 pets tagged Snow on the wiki that puts Mammoth at number 4 — Ice Dragon tops the Snow at $65M/s. Remember the game multiplies this base by the size roll and by the mutation, so two Mammoths can earn very different amounts.

With a colour mutation and no size bonus, the same Mammoth would earn roughly: Silver ×1.2 ≈ $50.40K/s, Golden ×2.5 ≈ $105K/s, Rainbow ×3.5 ≈ $147K/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 +19.2k Speed, $4.2m, 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).