I like thinking of other kelpie designs than Kal, who is meant to be sort of an ugly duckling/skinny little freak compared to a "standard" kelpie, and even though he is an adult, his true-form is kind of not at full potential. Fae I imagine work on some weird timescale like that, based on experience and mental state. Where Kal sees himself as somewhat of a runt of a kelpie, something barred from the growth and experiences that his immediate family probably had as kelpies in their natural environment and therefore his kelpie form is small and somewhat thin, he also knows the very real adult pressures of rent, bills, taxes, and social interaction, so his human self reflects that.

I imagine kelpies who were born in their native lochs and rivers, knowing themselves to be fae or at least to be beasts that are magical spirits of water, have a much stronger, wilder poise to them. For this one, I wonder if it's ever had any contact with humankind at all - other than as prey, perhaps.

Not pictured, but kelpie are generally able to appear as "normal" horses with various levels of tells that can give them away, from their hooves being wrong, to their eyes, or their manes being a bit weedy, or their teeth being too sharp. A skilled kelpie can probably be pretty significantly convincing - perhaps this one would appear as an unusually confident but otherwise completely average black horse, if you saw it roaming the wild. Only once you've touched its mane and been wrapped in thick dark waterweeds would you see it for what it is, and by then a watery grave would be close behind. Unlike Kal, this kelpie has not endured the traumatizing humanity of working food service to afford an apartment and hang out with friends to play gamecube. Rest easy in knowing you could at least beat it in Melee.