Miracle has the potential to heal 98 life, or if you use shards/stone skin miracle can heal as much as 518 life, but that's not a common scenario. Most of the time, miracle heals from 60 to 90 life. This card can heal a maximum of 49 life in one turn. Let's say the average is 25 life per use.
Now, let's go with a high estimate of Miracle, and say it heals 90.
That means that with just 4 uses of Divine Pony's ability, you have healed around 100 life, but with 1 Miracle you have healed 90 life.
And you still have the Divine Pony to use again and again.
The fact that it is a creature with an ability makes if vastly superior to a single-shot spell, even if it's ability is only a fraction of the comparative spell.
As an example, what's better? 1 Golden Hourglass, or Upgraded Precognition? Obviously the hourglass is better, but the cost for drawing 1 card with Golden Hourglass is 6
(4 to put it into play, 2 to use the ability) while Upgraded Precognition costs
. The fact that golden hourglass remains in play and can be used every turn makes it much more powerful than precog, even though it costs 6 times as much.
This card is not underpowered. It might be overpowered.
Jallen