The plural of "heir" is "heir". There are many other words like this. Is there any rule how we can understand when a word cannot have a different plural form? Or do I have to mug up those words? Please help me.
There is no rule determining whether or not a noun's plural form is the same as its singular form. As the comments point out, many of these words are words for animals. Another trend I have noticed is that loanwords from Japanese often don't change in the plural (two kanji, three Pokemon, ten anime), though sometimes they do (six ninjas, eighty futons).