Pterodactyl

Pterodactyl hatches from the Pterodactyl Egg, a Legendary-rarity egg found in the Prehistoric. Prehistoric is zone 8 of 11 on the normal route; the wiki lists its entry requirement as ~17M–18M speed, and the nests there are watched by the T-Rex.

The wiki lists its base income as $22K/s, which ranks it 7th of the 8 Prehistoric pets by base rate — Mosasaurus tops the Prehistoric at $180M/s. Base rate is what a normal-size, un-mutated hatch earns; everything above that comes from the size roll and the colour mutation.

With a colour mutation and no size bonus, the same Pterodactyl would earn roughly: Silver ×1.2 ≈ $26.40K/s, Golden ×2.5 ≈ $55K/s, Rainbow ×3.5 ≈ $77K/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 +270k Speed, $2.2m, and the egg takes 3m 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.