I really like when OctoJohn starts out teacup-sized and stays that way for a few months, but then suddenly hits a point where he starts to grow at the rate of a great dane.
He would start out in a 100-galleon aquarium that he would periodically escape from regardless of the lid, locks, or weights Sherlock put on it, and go raid Sherlock’s jam supply. He’d fit in teacups, be able to nestle fully in Sherlock’s palm, or ride up on his shoulder until he needed to get back to water. Sherlock doesn’t often let him wander, worried about the tiny thing getting hurt. If John is too eager for an adventure, Sherlock has a special container to take him around in. John doesn’t like this much, preferring to experience things without barriers.
Then one day, he finds that John has doubled in size. The next day, he’s the size of a basketball and has to be transferred to a larger tank. Even though he’s heavier than a toddler, he still tries to climb up and cling onto Sherlock. After a week, he’s practically fully grown, and Sherlock is split between keeping him at the research facility or aquarium, or returning him to the sea where he won’t see him again.
