If "Horseytail" is declared with "Prohibition" after the 'Horsetail Token' is Special Summoned, the discard effect is applied when 'Horseytail Token' is destroyed.
If a 'Horseytail Token' is Special Summoned onto your side of the field by the effect of your opponent's "Horseytail", when it is destroyed by battle with a Plant-Type monster and you send a card from your hand to the GY, if you send "Elephant Statue of Disaster" to the GY, its effect does not activate.
Sending 1 card from your hand to the GY when 'Horseytail Token' is destroyed by battle with a Plant-Type monster does not start a Chain.
If "Remove Brainwashing" or "Owner's Seal" is activated, control of the 'Horseytail Token' is shifted to the player that activated the effect of "Horseytail".