Yes, while all tilesets are made with 16x16 px tile resolution in mind, there are 16x16, 24x24, and 32x32 sprite sizes to keep animals proportion as normal as possible. Larger animals such as moose and elephants of course have larger sprites (32x32) to comply with their natural size. Otherwise you will have strangely proportioned animals.
If you use RPG Maker, I don't think there are any issue on using those sprite sizes as long as you use always use 3x4 sprite layout (please see the animals screenshots, all of them are made with RPG Maker).