Thank You very much according to rikaichan 匹 - き is a counter for rolls and small animals while 匹 - ひき is for horses or rolls of cloth. It turns out that monsters are either rolls, horses, cloths or small animals... Should stick to the latter I think.
Thank You once more.
Did you mean to write the same kanji twice up there?
The counter for horses is 頭 (とう）。 That works for big animals.
匹 will be read ひき if indeed it is the counter for monsters (which I am not disputing, merely saying I have no idea, as usually I am too busy running away from them to bother counting...)
The most common usage for 匹 is for animals, especially smaller ones.
Dictionaries do say 匹 is used as cloth (two rolls/二反 nitan makes 一匹） but it is not common now, at least not used/heard in today's daily conversation.
Dictionaries also say 匹 is used for horses, but today 頭 is most commonly used to count horses. It seems that in old days (centuries ago) 匹 used to be used to count ox/horse and later for small animals as well, but today big animals such as horses, elephants, whales are usually counted with 頭.