The Hippo and Archer ability doesn't work if you're trying to merge two units of the same type if I remember correctly. Back when I made this kind of ability for my Theramore Arena map, I had to trigger it.
The way I did it was to give the unit a single-target ability with a low cast range. When the ability was cast, I'd use a trigger to remove the caster and the target and place a new unit in their place.
I used another trigger to detect the order for the ability and cancel it in case the target was not a valid unit type. I also detected right-click ("smart") orders and if it was a valid unit type for merger, I'd issue the spell order instead.
i did some testing and found out that it does work without triggers and with 2 units of the same type. what you likely did wrong in your map may be the method. the abilities comes as a pair you see. mount hippogryph triggers pick up archer and vise versa. as a result the unit needs both if a unit shall merge with another unit of the same type. as i got what i wanted you are welcome to close this topic but please tell me how you messed up in your own map.(i just want to know.)
i usually figures stuff out myself. i work on warcraft 3 maps about every day when i am working at maximum but, i only come in here to ask a question about once a month.
As I recall, I assumed that if the unit type was the same, the ability could trigger up itself on the other unit, so I only tried with a single ability. I didn't consider the possibility of giving a unit effectively two identical copies of the ability.
it is 2 different abilities that work together to trigger an effect. mount hippogryph and pick up archer triggers each others. i just tested 2 copies of mount hippogryph on the same unit and then it failed. when however i gave the unit both mount hippogryph and pick up archer it worked. do you understand now?
