Although coming at a very close joint second are probably the two most dangerous and intelligent animals known to man. Rats or Rattus norvegicus have an impressive intelligent vs brain size ratio that if they were of equal size to me or you and played chess against us they would win. Not only would they beat us at chess but whilst playing that game, their soldiers of super strength and agility would have secretly wiped out our abilities to combat them. Yes they would be capable of under-handed dirty tricks, something that we pride ourselves in.
Let me play you the song
of my people
With rats however we have the benefit that some of them are domesticated and make brilliant and loyal pets, even more than cats and dogs (look it up!) So there is a chance we could get some evil rat general being vanquished by a loyal female rat who was in love with its human owner but sacrificed herself so he could live, immortalizing the fight for equality amongst our races. Which is kind of a good thing because a rat and human baby would be ugly.
Only a mother's love
On the other hand, the hand which there is no escape and no chance of victory, we have the Octopus or Octopus Vulgaris. Forget sharks, forget bears, forget spiders, if there is one animal, one creature that we need to be afraid of it is the octopus. On a level of one to terrifying these creatures are a 'Shit your pants.' The amount of things that are going for these beasts from the depths is ridiculous.
Again their potential intelligence is comparable to the size of their brain. It's the size of a peanut yet they show incredible problem solving capabilities, memory and all around scary stuff. Just read this article and start building your fortifications, not that they will help. There has been some evidence that Octopus have actually tried to come ashore, dying shortly afterwards of course. Don't you see! The invasion is beginning they are just the scouts, testing our defences, the war isn't coming it has already arrived!
Supplies for the effort
On the plus side most Octopus only live for about one or two years so we can just play the waiting game. This is probably the major reason why they haven't evolved or went beyond the level they are at because they can't build on the intelligence they are all inherently born with. If they start living ten or twenty years then it's time for us to step down from our pedestal.
If this happens hopefully the civil war between Rat and Man has ended and we have entered an alliance. Only with our combined intellect could we defeat the beast of the deep! Imagine riding rats into battle against flying Octopus, be sick wouldn't it!
Couldn't find a picture of that so here is another scary octopus picture: