Fennec

The Fennec Egg is the Uncommon-rarity egg of the Desert, and the pet inside it is Fennec. Desert is zone 3 of 11 on the normal route; the wiki lists its entry requirement as 10K speed, and the nests there are watched by the Scorpion.

Base income is $18/s. Among the 8 pets tagged Desert on the wiki that puts Fennec at number 7 — Royal Sphinx tops the Desert at $280K/s. Remember the game multiplies this base by the size roll and by the mutation, so two Fennecs can earn very different amounts.

With a colour mutation and no size bonus, the same Fennec would earn roughly: Silver ×1.2 ≈ $21/s, Golden ×2.5 ≈ $45/s, Rainbow ×3.5 ≈ $63/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 +8.1k Speed, $1.8k, and the egg takes 30s 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.

Uncommons are cheap speed: hatch them for the reward, then move on.