Arantulas are found only in the Pit of 100 Trials in Levels 81-89. It has the same attacks as the Pider, but it's more powerful. Mario can only hammer it when it goes down, but not when it's up, unless he's equipped with the Hammer Throwbadge. He can jump on it to damage it, but the best attack to use against it is the Star PowerArt Attack to draw around it to kill it quickly.
Arantula is the word "Tarantula" without the letter "T".