Chimera

The chimera is the son of Echidna, the mother of all monsters.

In The Lightning Thief, Percy is cornered by a fat lady and her chihuahua at the top of the St. Louis Gateway Arch. The lady constantly refers to her pet as "sonny" or "son" and it is only when Percy is forced to wait for a second elevator down and left without the company of Grover and Annabeth that he discovers who the fat lady really is.

The chihuahua transforms into his true form, that of the chimera and attacks Percy. As Percy tries to defend himself and a few innocent bystanders with Riptide, he is outwitted by the creature and ultimately forced to dive out of the Gateway Arch (via a hole conveniently provided by the chimera) into the muddy Mississippi.