If a pet is following you, it will mount a Happy Ghast when you do

1) This would only apply to Tamed Wolves, Cats, and Parrots. (If other following pets are added that you can make sit, this would be carried forward)

2) When they mount the ghast, the will take up one of the available seats on the ghast harness, as such the player can only take three. Any other pets would have to be placed in a boat as usual.

3) When dismounting, the player will have to manually interact with the pet to get it to stand, and then it will resume following you. (Parrots will default to mounting you, instead)

4) (optional) A method to swap seats could be devised such that the pet may occupy the "driver's seat" of the Happy Ghast. When this happens, and you are not on the ghast, the Pets' following AI will be used to "drive" the Happy Ghast for you, as long as you are above Y = +56. This includes teleporting to the player. The Happy Ghast would still be tempted with Snowballs as an override however. Also, the Happy Ghast will not try to go places it will not fit.