From AMule Project FAQ
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Mule clients are those P2P clients which connect exclusively to the ED2K network (and possibly to the Kademlia network).
It can be understood more easily as "those clients which pretend to be clones of the eMule client" allthough they might extend its features and lack others.
Usually this clients have names such as eMule, aMule, xMule, lMule, jMule, etc (that's why they are also popularily known as *Mule clients), but some have very different names such as lphant.