A party horn is a horn formed from a paper tube, often one that is flattened and rolled into a coil, and which unrolls when blown into, producing a horn-like noise. The item is not known consistently by any term in English, also being known by a number of local variations, neologisms, and individual terms, often containing variants and synonyms of blowing (puffing, blow-out etc.) and noise (whistle, squeak etc.)
Modern variations have a plastic mouthpiece, which prevents the swift degradation of the device from exposure to the moisture of the mouth. Often the paper tube contains a coiled up metal or plastic strip that rapidly retracts the horn when you stop blowing on it. Others have a brightly colored feather attached to the end which vibrates in the outgoing air flow as the horn is blown.
Look up party horn in Wiktionary, the free dictionary.
Some people distinguish between different types and are quite specific with the various type:
party horn, screamer, tweeter, squeaker, noisemaker - shaped like a horn, makes some sort of a tooting noise
noisemaker - usually made out of tin, a small box with a handle on it, when spun around it makes a clicking noise
clapper - has two parts on it, usually the same shape, when waved back and forth, makes a clapping noise