Swordfish

Swordfish (Epic rarity) belongs to the Abyss Ocean zone. Abyss Ocean is zone 7 of 11 on the normal route; the wiki lists its entry requirement as ~2,5M speed, and the nests there are watched by the Moby. You get it by carrying the Swordfish Egg back to your pen.

At $1.1K/s base income, Swordfish sits 7th out of 8 in its zone — El Maja tops the Abyss Ocean at $130M/s. The listed figure assumes no mutation and a normal weight.

With a colour mutation and no size bonus, the same Swordfish would earn roughly: Silver ×1.2 ≈ $1.32K/s, Golden ×2.5 ≈ $2.75K/s, Rainbow ×3.5 ≈ $3.85K/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 +81k Speed, $110k, and the egg takes 2m 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.

Epics are the mid-tier of their zone — a good candidate for your first mutation roll because the base rate is high enough to matter.