Unstable abyss-bat

Unstable Abyss-Bats are magically-enhanced, level 99 monsters found in the Magic dungeon north of Varrock.

They are the second highest-level non quest, non random event and non-dungeoneering monster in free to play Runescape. They are notable for being very weak for their level.

Other than the Giant Rats, they are the only monsters in the three-level dungeon. Due to their low max hit of 26 but rather high health and life-recovering abilities, they are extremely popular for training. They are also the only monsters in the game to drop the much-coveted Silly Hat, one of the most powerful and expensive weapons in F2p.

Trivia
When first released, players fighting Gigantic bats on the second level of the dungeon noticed that they would sometimes drop into the floor, with only their ears showing. This led to a programming glitch wherein the ai of player characters would be unable to 'locate' the bat it was fighting, causing it to run around without attacking, being chased by the floor-bat. This bug was not fixed for two days and became known as the "Floor Shark" glitch. Jagex later payed homage to it by releasing the Abyssal Shark random event in the Abyss.