question about creat unit

I have a question about the unit living time.

I want to creat a unit just like Orc's spirit wolf that it will disappear in a fixed time.
I've tried to use the wolf to be my model. But it won't work.
The unit still lasts and won't disappear.

Is it possible to use WE(unit editor) to create a unit like this?

and if I have to use trigger to correct this problem, how do I do that.

thanks for your reply.