Thinking About Another Old Book

Once more ancient memories have been teasing my brain. I don't know if it's the season, or what, but I've been remembering a couple of old stories I read back in third grade or so, stories whose name and author elude me.

The details I recall are few. As I remember it, these were animal stories where the animals acted like humans, but were drawn in a very realistic manner. One of the books was mainly about mice; the other had more of a mix of creatures.

In the latter tale, a group of homeless animals come across an abandoned castle or great house. They decide they will live there, pretending to be its lord's servants. I remember a mouse in fancy dress, carrying a ring of keys, saying something like "I am his lordship the [something like the Duke of Carrabas]'s Master of Keys." There was also a song sung (by the pretend minstrel, I suppose) called "The Terrapin Turtle of Zanzibar." It began:

"The Terrapin Turtle of Zanzibar
Rode a tricycle and smoked a cigar..,"

There was an illustration of this.

All searches so far have turned up nil. If anybody out there has a clue, or knows what I'm talking about, I would appreciate any information you could supply.