A Viking Yeti
Carnal Leviathan
You can't let them use transports, will freeze the game. Unless you mean land transports, which the AI can't use. Also, I think you should have Hid. Nat. and invisible, so the AI wouldn't be able to go straight for them... ...Unless you have other units who see invisible units, in which case this may be a problem. I would give them Hid. Nat. simply because you don't have to be at war with someone for your landmines to still kill people (also angering the country you placed them in).