Recall training is a race between your voice and everything more interesting than your voice. A short name gets there first. Four letters or fewer and no more than two syllables is the definition used here, and it is computed from the spelling, so nothing in this list is short by opinion.
Why short works
Two reasons, and only one of them is about the dog. A short name is quicker to say, so you say it sooner and more often. And a short name is harder to soften: nobody stretches a one-syllable name into the sing-song that turns a recall cue into background noise.
Short and unisex
Useful before you know which animal is coming home, or when the shelter's guess about sex turns out wrong.
Every count on this site is a tally of real registrations: pet licences and shelter intakes published as open data by New York, Seattle, Austin, Montgomery County, Cambridge, Zurich, Basel and three Australian councils. Popularity here is the average share a name holds across those registers, so a name common in a small city is not buried by one common in a large one. Nine of the eleven sources are North American, so read the ordering as naming in those places rather than worldwide.