cat.txt
dog.txt
house.txt
tree.txt

file1.txt
file2.txt
file3.txt
file4.txt

rename -n '$_ = sprintf "file%d.txt", ++$::count' *

$ c=0;for f in *; do ((c++));  echo "$f" "--->" file"$c".txt ; done
cat.txt ---> file1.txt
dog.txt ---> file2.txt
house.txt ---> file3.txt
tree.txt ---> file4.txt

$ c=0; while IFS= read -r f; do ((c++)); echo "$f" file"$c" ; done < input.txt
cat.txt file1
dog.txt file2
tree.txt file3