Yes, it is possible for animals to visit "spiritually" (probably in the etheric, which actually is a higher level of the physical plane).
My son had a dog called Tory, who used to enjoy the occasional visits with us. Eventually my son left for overseas, and a few weeks later the dog (who was in the care of friends) got sick and died. We knew, because Tory turned up in our garden, and said this was the most comfortable place for her to be, and that she would move on in a few weeks... which she eventually did. We didn't hear her bark or anything, but my late wife was clairvoyant and could see and talk to her.

Cheers -
Pounamu