𝖶𝗋𝗂𝗍𝗂𝗇𝗀 𝖳𝗂𝗉𝗌 𝖿𝗈𝗋 𝖫𝗈𝗌𝖾𝗋𝗌

𝖥𝖺𝗇𝗍𝖺𝗌𝗍𝗂𝖼 𝖡𝖾𝖺𝗌𝗍𝗌 𝖠𝗇𝖽 𝖧𝗈𝗐 𝖳𝗈 𝖶𝗋𝗂𝗍𝖾 𝖳𝗁𝖾𝗆 (𝖨𝖵)

❖ 𝗖𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝘁𝗮𝗶𝗹𝘀

✎ 𝗢𝗻𝗹𝘆 𝗺𝗮𝗸𝗲 𝗮 𝘁𝗮𝗶𝗹 𝗶𝗳 𝗶𝘁'𝘀 𝗮𝗰𝘁𝘂𝗮𝗹𝗹𝘆 𝗯𝗲𝗻𝗲𝗳𝗶𝗰𝗶𝗮𝗹 𝘁𝗼 𝘁𝗵𝗲 𝘀𝘁𝗼𝗿𝘆
Unlike what you may think, tails are very, very tricky to write about if you're putting them on humans, which I assume is what [redacted] is asking about. There's something about them that makes it incredibly difficult to describe them without making it seem silly. (Unless your character is a mermaid / has a tail that's used for swimming; in that case, it's reasonable, which is exactly what you want- for it to be reasonable).
If your fantasy species/character has a tail, make sure that it is ABSOLUTELY NECESSARY, because otherwise you risk your readers finding it ridiculous, especially if the tail is just used for comic relief like annoying other characters (I'm not saying that you shouldn't take out the tail if it's used for comic relief, I'm just saying if that's the only thing that the tail does, you might want to reconsider giving your character one). 

Things that your character's tail can be used for:
✎ 𝗕𝗮𝗹𝗮𝗻𝗰𝗲
✎ 𝗕𝗮𝘁𝘁𝗹𝗲
✎ 𝗛𝗼𝗹𝗱𝗶𝗻𝗴 𝘁𝗵𝗶𝗻𝗴𝘀
✎ 𝗥𝗲𝗹𝗲𝘃𝗮𝗻𝘁 𝘁𝗼 𝘁𝗵𝗲 𝗰𝗿𝗲𝗮𝘁𝘂𝗿𝗲 𝘁𝗵𝗲 𝗰𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿 𝗶𝘀 (𝗹𝗶𝗸𝗲 𝗮 𝗱𝗿𝗮𝗴𝗼𝗻)
✎ 𝗣𝗼𝗿𝘁𝗿𝗮𝘆𝗶𝗻𝗴 𝗲𝗺𝗼𝘁𝗶𝗼𝗻
✎ 𝗦𝘄𝗶𝗺𝗺𝗶𝗻𝗴

✎ 𝗞𝗻𝗼𝘄 𝘁𝗵𝗮𝘁 𝘁𝗵𝗲𝗿𝗲 𝗮𝗿𝗲 𝗱𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝘁 𝘁𝘆𝗽𝗲𝘀 𝗼𝗳 𝘁𝗮𝗶𝗹 𝘁𝗵𝗮𝘁 𝘀𝗲𝗿𝘃𝗲 𝗱𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝘁 𝗽𝘂𝗿𝗽𝗼𝘀𝗲𝘀
We seem to be one of the few animals that lack a tail. All animals have it, and therefore that means there are plenty of kinds of tails for you to choose from, both real and imaginary. Here's a list:
✎ 𝗕𝗶𝗿𝗱
✎ 𝗕𝗼𝘃𝗶𝗻𝗲 (𝗰𝗼𝘄, 𝗼𝘅, 𝘄𝗶𝗹𝗱𝗲𝗯𝗲𝗲𝘀𝘁, 𝗲𝘁𝗰)
✎ 𝗖𝗮𝗻𝗶𝗻𝗲 (𝘄𝗼𝗹𝗳, 𝗱𝗼𝗴, 𝗱𝗶𝗻𝗴𝗼, 𝗲𝘁𝗰)
✎ 𝗖𝗲𝗿𝘃𝗶𝗻𝗲 (𝗱𝗲𝗲𝗿, 𝗲𝗹𝗸, 𝗰𝗮𝗿𝗶𝗯𝗼𝘂, 𝗲𝘁𝗰)
✎ 𝗘𝗾𝘂𝗶𝗻𝗲 (𝗵𝗼𝗿𝘀𝗲, 𝗽𝗼𝗻𝘆)
✎ 𝗙𝗲𝗹𝗶𝗻𝗲 (𝗵𝗼𝘂𝘀𝗲𝗰𝗮𝘁, 𝘁𝗶𝗴𝗲𝗿, 𝗹𝗶𝗼𝗻)
✎ 𝗺𝗲𝗿𝗺𝗮𝗶𝗱/𝗺𝗮𝗻
✎ 𝗠𝗼𝗻𝗸𝗲𝘆
✎ 𝗥𝗮𝘁/𝗺𝗼𝘂𝘀𝗲
✎ 𝗥𝗲𝗽𝘁𝗶𝗹𝗶𝗮𝗻 (𝗱𝗿𝗮𝗴𝗼𝗻, 𝗶𝗴𝘂𝗮𝗻𝗮, 𝗲𝘁𝗰)

✎ 𝗨𝗻𝗹𝗲𝘀𝘀 𝗶𝘁'𝘀 𝗺𝗮𝗴𝗶𝗰, 𝘁𝗵𝗲 𝘁𝗮𝗶𝗹 𝗵𝗮𝘀 𝗶𝘁'𝘀 𝗹𝗶𝗺𝗶𝘁𝘀
Unless specified, your character's tail can't do loop-de-loops and stretch to crazy lengths or whatnot.
Some tails are powerful and muscular, capable of doing some hefty destruction (aka Reptilian and Mermaid) and others are dainty and delicate, more of a nuisance in battle than an actual weapon (Feline).
Like I said, tails have different jobs, and if you have a dog tail doing a monkey tail's job, you're gonna have some serious backlash from the readers, who will either critique harshly or simply stop reading altogether, and that's the LAST thing you want.
If after this you don't heed my warning and DO YOUR RESEARCH PLEASE!, I can guarantee that your readers will not be a fan of your character's tail, even if you think you know everything there is to know about it.

✎ 𝗨𝗻𝗹𝗲𝘀𝘀 𝘆𝗼𝘂𝗿 𝗰𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿 𝗶𝘀 𝗮 𝗺𝗲𝗿𝗺𝗮𝗶𝗱, 𝗮 𝘁𝗮𝗶𝗹 𝘀𝗵𝗼𝘂𝗹𝗱𝗻'𝘁 𝗯𝗲 𝘁𝗵𝗲𝗶𝗿 𝗼𝗻𝗹𝘆 𝘁𝗿𝗮𝗶𝘁
"Oh yeah, I'm a dragon but the only thing I have is this tail."
"Lit."
"Yeah, just like your home with your family in it rn."
If you're writing about a fictional species, as a reader I am begging you not to just put the tail as a defining feature. Otherwise, your creature is gonna feel pretty lame.