Welcome to the board.

One way:

Row\Col
A
B
C
D
E
1
Horse
Prob
Cumu
2
 Alan
12%
0%
 C2: =SUM(B$1:B1)
3
 Barb
14%
12%
4
 Cain
5%
26%
5
 Dana
19%
31%
6
 Eric
9%
50%
7
 Fran
22%
59%
8
 Gary
14%
81%
9
 Hana
5%
95%
10
11
 Gary A11: =INDEX(A2:A9, MATCH(RAND(), C2:C9))