Re: what is this mount called,and how hard is it to replace..2000 mercury 90hp
That is the lower point where the mid section of the engine connects to the bracket attached to the pivot tube which is attached to the transom mounting bracket assy. Used to be soft rubber to isolate engine vibrations from the boat, but after the advent of PTT and bass boats running 65 and all OEM's found that the mounts had to be firm. So I don't know what you'll find.
On difficulty, you may be able to replace them without disturbing the upper ones. Otherwise, if you have to pull the engine to get both top and bottom off the studs then you will have to have a hoist to support the engine.
Why do you need to replace them?
Mark