Thanks for your answer. I find it strange that they can swim but don't have a swimming animation like all other animals. It looks so weird seeing the ostrich walk like that inside the water without touching the ground.
I've never witnessed it myself in-game so I couldn't say whether it's a one-off occurrence in your game or whether it is just missing swimming animations. Maybe observe it for a while and see if it keeps happening and make a bug report for it?
That said, hopefully they'll just remove the swimming entirely instead of creating or correcting the animations for it. I already hate the way the antelope seem to run directly for the water in-game even though their natural instincts would turn them away from it. It's a case of "they can swim, yes, but they typically wouldn't unless it was incredibly necessary" (same for the ostriches, and lions - in real life whether a lion swims or not tends to depend entirely on the cat's personality). I don't know enough about cassowaries to say either way whether swimming is a common behaviour in them, but I wouldn't be inclined to think it was considering they have no natural protections against the perils of water.