Obviously, this book is all about nature—it's called The Secret Garden, for Pete's sake. So it should come as no surprise that even the animals mentioned briefly have a symbolic value in the book. When Mary's parents die and leave her alone without her even knowing it, she spots a snake in the house:
But no one came, and as she lay waiting the house seemed to grow more and more silent. She heard something rustling on the matting and when she looked down she saw a little snake gliding along and watching her with eyes like jewels. She was not frightened, because he was a harmless little thing who would not hurt her and he seemed in a hurry to get out of the room. He slipped under the door as she watched him. (1.23)
This snake isn't a sign of evil or anything like that—the bungalow isn't the Garden of Eden—but his eagerness to "get out of the room" is both realistic, since not many animals love human company, and symbolic. The snake is like a stand-in for Mary, totally alone in the world and not at all at home in the bungalow that belongs to Mary's parents.
When Mary finds herself alone in Misselthwaite Manor, on the other hand, she finds a family of seven tiny mice nesting in one of the couch cushions. As Mary looks at the mice (six babies and one mother), she thinks:
If there was no one else alive in the hundred rooms there were seven mice who did not look lonely at all. (6.28)
Where the snake seems to echo Mary's general sense of being out of place and lonely in her parents' empty bungalow, the happy colony of mice appear to suggest that Mary is going to find a community, even a family, during her time at Misselthwaite Manor. Even though Yorkshire seems so rural and bare compared to what Mary is used to, there are actually a lot more possibilities for her to make friends here—Dickon, Colin, and even Ben Weatherstaff are going to save Mary from feeling lonely or out of place again.