This is the card your allies have been waiting for!
In the spirit realm I would rate it as one of the strongest allies for its cost. On it own it's not steller but once you use its action to attach to another Ally you really get to see how good this horse can get. Counting as restricted means Ents and Eagles are out, but the rest of the Ally pool is wide open and fair game.
It's best paired up with already strong allies to push them into hero-stats levels and even better with ones that have some readying effects (or paired up with Leadership Faramir (or valiant determination attachment) to be able to use their cross-boosted stats multiple times in a round.
My favourite targets are Hobbit ally Gandalf who becomes a 5/5/5 monster stronger than any other hero that can quest and battle since he doesn't exhaust to quest. Other decent ones are Boromir, Gimli and Wiglaf who all benefit from the boosted stats and built in readying.
Hope you enjoy this excellent steed for our allied friends!