"Animals are also boarding the ship"
This (or similar) should be the default translation. Getting on / getting in / boarding are all equivalent in this case. However, "getting in" seems least likely as you would "get in" a small boat, but "get on" or "board" a large boat or ship.
The "too" at the end of the sentence is too repetitive for Duo to so frequently use, and also unnatural in this case.