[GotaFancy?]

So if a horse in one sheet is listed in another sheet, then you want to say "Yes", otherwise leave it blank?

Looks like a vlookup may be your best bet, combined with an ISERROR something along the lines of:

=IF(ISERROR(VLOOKUP(A2,Sheet2!A:A,1,FALSE)),"","YES")

If your list of horses is:

B

G

H

Then this gives this output:

Horse
A
B	YES
C
D
E
F
G	YES
H	YES
I
J
K
L

Is that what you're after?