Fenrir had been undergoing a growth spurt, but it quickly learnt that the larger it grew, the more obvious its existence became to the bossy turtle. As such, he had been trying hard to repress his growth. He was not able to do that, but by focusing so much on a specific thing, he was able to unlock a bloodline ability that let it manipulate its size.
These days, it spent most of its time either the size of an ordinary dog or the size of a pup. It was unfortunate, since the larger he became, the more love the various guests would give him, which was nice. But the turtle was too scary, he had to avoid it.
As for what made it scary? No matter how Fenrir displayed its bloodline repression, the turtle would remain unaffected. At least in other beasts, this was not an ordinary reaction. Even the grumpy dragon atop the mountain gave him at least a snort when he used his repression.
