The phrase "is tail" can have a few different meanings depending on context, but there are several synonyms that could be used in place of it.
In terms of animals, you might use terms like "has a tail" or "sports a tail" to describe an animal's physical feature. You could also use "tail end" to refer to the back or last part of something.
In a more figurative sense, "is tail" might be used to indicate a trailing or subordinate position. In this case, synonyms could be "is at the bottom" or "comes last."
Ultimately, the synonyms used would depend on the specific context and intended meaning of the phrase.