This is a hunter's primary method of healing his or her pet. (Other ways include First Aid and Gift of the Naaru.)
As noted below, this is an instant cast, heal over time spell. It heals in 5 ticks over a 15 second period. Specifically, it provides 20% of the heal amount on each of the 3rd, 6th, 9th, 12th, and 15th seconds.
Mend Pet does not scale, it heals for the same amount regardless of attack power or spell power.
With Improved Mend Pet, the spell has a high probability (about 76% at Rank 1 and 97% at Rank 2) of removing at least one debuff by the fifth tick.
Glyph of Mend Pet gives Mend Pet the additional effect of improving the pet's happiness slightly.
Glyph of Mending increases the healing done by Mend Pet by 3%.
Blood of the Rhino increases all healing received by the pet including Mend Pet by 40% for two tenacity pet talent points.
Spirit Bond increases all healing received by the pet including Mend Pet by 10% for two talent points.
All improvements stack by multiplying for a possible maximum of 103% * 140% * 110% - 100% = 58.62% increased healing.
Before Patch 2.1.0, this effect was channeled over 6 seconds, cost more mana, and healed for less. The patch changed it to an instant effect that costs much less mana and heals for a larger amount, but much more slowly. Mend Pet now enables Hunters to resume dealing dps as it no longer requires channeling.