Three short lines, and almost nothing happens. A dog that was running is not running anymore. What makes the poem work is the phrase "no longer," which tells us the running used to be true. We are looking at a change, and the poem never says what caused it.
That gap is where the feeling lives. The dog might have simply stopped to rest, and the mood would be calm. But the flat, quiet wording also allows a darker reading, that the dog has died or grown too old to run. The poem doesn't choose for us. The speaker sounds restrained, almost hushed, as if noticing the stillness is enough, or as if saying more would be too much.
The short lines and the white space around them add to this. Each break is a small pause, so the reader slows down and the sentence seems to settle, much like the dog. What's left is a mood of quiet, loss, or gentle acceptance, depending on how you read it.