Did you ever wonder why there are no insects or spiders sized as big animals? The answer lies in their respiratory system and hemolymph (blood). In humans and other animals, body cells get oxygen through blood cells that capture oxygen from the lungs. Insects' blood (hemolymph) does not carry oxygen, and body cells get it directly from the air that is supplied by the complex system of little hollow tubes (called tracheas or windpipes). Since insects don't have lungs but only "passive" tracheas, such primitive "ventilation" system can only stay effective as the length of tracheas is short enough, otherwise the "deep down" cells will not be ventilated enough and will die of oxygen starvation. That is why those scary old Japanese movies with huge insects (gigantic moth shooting thunderbolts from its eyes, etc) fighting giant animals are physically impossible in real life.
If insects had some features of animals (lungs, etc.) huge size would not a a problem for them. So today, let's combine any insect(s) with any animal(s) any way you wish. (For the sake of this contest we'll consider spiders as insects, though formally belong to separate order Araneae).
Many thanks to johnx1 for the themepost.