Warhorse — Timothy Zahn

The aliens were pacifists -- but all living things were their friends...

When humanity first ventured out among the stars, it seemed that we were alone in the universe, the only tool-users in the galaxy--and thus were free to mold the very cosmos to our heart's desire. It would take us a long time--our star drive wasn't very fast--but it was our Manifest Destiny ... and, besides, there was no one to stop us.

Then we met the Tampies. In the arts mechanical they were less skilled than we, but in the biological sciences they were incomparably more advanced. Either as a result of or because of that advancement, the Tampies held all life to be part of a single pattern, and themselves to be its guardian: to harm the slightest part of it was to harm the whole, and that they would prevent at all costs.

Humanity being what it is, it was inevitable that such a species ethic would bring its holder into conflict with Man. Unfortunately for us, the Tampies' living spaceships were much better than our metal ones: the Tampies had Warhorses....