What is the meaning of Squealer?

Any animal or person who squeals.

A pig.

An informant.

A kind of jug used to carry beer having 1 litre or 32 ounce capacity (half the size of a growler).

Anything that makes a squealing noise.

The European swift.

The harlequin duck.

The American golden plover.

A young pigeon.

Source: wiktionary.org