Echo was introduced in the Tempest block and has been used only a handful of other times, such as during the Time Spiral block.

702.27. Echo

• 702.27a. Echo is a triggered ability. "Echo [cost]" means "At the beginning of your upkeep, if this permanent came under your control since the beginning of your last upkeep, sacrifice it unless you pay [cost]."
• 702.27b. Urza block cards with the echo ability were printed without an echo cost. These cards have been given errata in the Oracle card reference; each one now has an echo cost equal to its mana cost.

Translation:

• 702.27a. - At the beginning of your upkeep, pay the described price or sacrifice the creature.
• 702.27b. - Cards from the Urza block were printed without an Echo cost, at the time their casting cost was the echo cost. It was later changed so that a cost is put next to the ability, this provides more flexibility for the game designers and follows the standard for most other costed abilities such as Kicker.