Amphibian is one of the three skins released in one part of the Jump Scares offer. It could be purchased on an individual basis using game coins in early November 2021.
Amphibian could be purchased in the Skins Shop under the 'Jump Scares' tab for 1390 coins. It was available for a limited time and may not return in the future.
Details of this offer can vary depending on location, platform or time.