Retexturing someone to bison is possible. Not really or sure about whenever you can change the shape to make it less block-y looking. Resource pack can't change mechanics and technical features of game, so no - you can't make cows fly just only by resource pack. However - changing the ghast to look like bison would someway accurate enough (coming back to the first problem - if changing the shape isn't the issue).
Then you'll have go along with that and pretend it doesn't do that, or just retexture the cows/sheep to look like bisons and imagine that they are flying (or just create a plugin that allows you soar with 'em).