As in the word 'Tough', that would mean to me the strongest animal. Well if you mean the
strongest animal within fighting, that's the answer I will give. Personally, I think the
strongest animal on land is an African Elephant and in water a Blue whale. However, if
they were to fight other animals, there is a very low chance that they could win. So a
snake such as a 'King Cobra' could kill an elephant in just three hours. But there are more
toxic snakes than that. The most Venomous snake is probably a black mamba of Africa
which it's venom can kill a human within 15 minutes. A Black Mamba would obviously be
able to kill an African Elephant quicker than an King Cobra, so the strongest animal
within fighting is probably the most venomous snake. I say the Black Mamba, but it
could be the Inland Taipan of Australia. If you can find out the most venomous snake
that would be the toughest animal.
Well, out of those two snakes, the Inland Taipan is the planet's most venomous snake.
But I think that the person that posted this question is wondering which animal is best at surviving horrific conditions. Despite it's size, Tardigrade Ecdysozoa, the binomial name for the microscopic organism known as the Water Bear, or otherwise known as the Moss Piglet, it can survive temperatures as low as -328°F and as high as 303°F, 570,000 Roentgens, the units used to measure radiation, 120 years without water, and it has been tested to survive the vaccum of space. I'll put my knowledge about strength later.