Hello; I cannot say from experience as I have not kept a horse nor a tank on a rubber mat, so you may be correct.
The gym I go to has had half inch rubber flooring put down. I am pretty sure when a machine is moved there is a depression left in the mat. Not much but present. The machines likely will not weigh as much as a tank big enough for an arowana. (that is a somewhat proper sized tank.)
The other thing is that horses do not stand on one spot for years. A tank can be in place for years.
Not that it makes a lot of difference but most horses I have been around weigh less than 2000 pounds. Some can such as the ones in the Budweiser ad's and a few others. Most are maybe 1200 pounds range. My second wife liked horses so I stopped at the Kentucky Horse Park a lot. They had a few 2000 + pounders.
Just throwing this in for the OP to consider.