Com o GNU sed:
$ sed '/^animal/IR File1' File2
...
Animal Name
This Dog
...
AniMal Type
This Cat
...
AnIMal Class
This Duck
...
animal Brand
This Horse
...
onde as extensões GNU são as seguintes:
The I modifier to regular-expression matching is a GNU extension which causes the regexp to be matched in a case-insensitive manner.
e
R filename
Queue a line of filename to be read and inserted into the output stream at the end of the current cycle, or when the next input line is read. Note that if filename cannot be read, or if its end is reached, no line is appended, without any error indication.