Shadow Dragon

Shadow Dragon is listed by the wiki as an Event pet (Event rarity). Event pets do not spawn in normal zone nests; according to the Eggs page they are only obtainable through the Store or special events such as Admin Abuse, so if you are looking for one in a biome you are wasting speed.

The wiki lists its base income as $25M/s, which ranks it 14th of the 23 Event pets by base rate — Mecha Dreadscale tops the Event group at $4B/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 Shadow Dragon would earn roughly: Silver ×1.2 ≈ $30M/s, Golden ×2.5 ≈ $62.5M/s, Rainbow ×3.5 ≈ $87.5M/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.

Neither a hatch reward nor a grow time is recorded on the wiki for Shadow Dragon yet; if you have hatched one, the timer on the pen and the reward pop-up are the two numbers worth noting.

Event pets are time-limited. If the event is over, the only route left is a player who already has one.