There are guide dogs and companion potbelly pigs and bomb-sniffing honeybees; there are dogs who will bring you your slippers and a dead squirrel, and cats who will bring you a dead bird; and all of these are very useful pets. But Saint Colman of Kilmacdough (Irish, 7th century) had three rather unusually useful helpers.
First there was the cock who awoke him early each morning to pray his night office.
Then there was the mouse who kept him from falling asleep while he prayed his night office.
Then there was the fly who acted as his bookmark, keeping his place in the missal.
How did the mouse keep him awake, you well may ask. The legend does not elaborate.