Hello and welcome to the forum scwilly.
Yes, you absolutely can create this filter. To find all occurences of cat in the field named Animal, create a filter named Cat with the following expression:
[font="courier"]Instr("cat",Animal)>0 /font[/quote]The Instr function looks for a specified string within another, in this case your Animal field. When it finds a match, it returns the position within the string where your search term begins. So when a match is found, the value is always greater than zero. No match? Return a zero.