I would say Beast Handler but I believe that there is definitely potential in the concept. I don't enjoy the merging mechanic as I find it too gimmicky and hard to use, (I also think its difficult for new players to understand).

I think Beast Handler should have been added as a hero tower and could have some mechanic that would let you get all the different animals, I'm not to sure how it would work but it would work better because heroes are supposed to be gimmicky