My first thought was Brian Quick. But then, I thought a little bit more about what a dark horse is;
A dark horse is a little-known person or thing that emerges to prominence, especially in a competition of some sort or a contestant that seems unlikely to succeed.
That would point me to this list of players;
# NAME POS. HT. WT. AGE EXP. COLLEGE
31 Alexander, Maurice S 6-1 220 23 R Utah State
82 Bayer, Alex TE 6-4 258 23 R Bowling Green State
26 Bryant, Christian S 5-9 198 22 R Ohio State
41 Bryant, Jarrid DB 6-3 185 24 R South Dakota
43 Cunningham, Avery DB 5-11 200 23 R Central Michigan
15 Franklin, Austin WR 5-11 189 21 R New Mexico State
33 Gaines, E.J. CB 5-10 190 22 R Missouri
5 Gilbert, Garrett QB 6-4 221 23 R Southern Methodist
68 Harlan, Deantre DT 6-4 281 23 R Bacone
Harris, Jordan WR 6-2 215 22 R Bryant University
51 Hill, Aaron LB 6-1 231 23 R Minnesota
45 Jones, Kadeem RB 5-10 266 22 R Western Kentucky
48 Millard, Johnny LB 6-2 232 23 R Cal Poly State
39 Reid, Greg DB 5-8 190 23 R Florida State
65 Rhaney, Demetrius C 6-2 301 22 R Tennessee State
47 Roberson, Marcus CB 6-0 191 21 R Florida
62B Salimu, Abasi T 6-5 310 24 R Nicholls State
96 Sam, Michael DE 6-2 261 24 R Missouri
17 Sherman, Jamaine WR 6-2 188 22 0 East Texas Baptist
3 Spencer, Diontae WR 5-9 161 22 R McNeese State
66 Van Dyk, Mitchell T 6-7 299 23 R Portland State
42 Watts, Trey RB 5-9 208 23 R Tulsa
62W Westbrooks, Ethan DT 6-4 267 23 R West Texas A&M
49W Wilson, Tavarius LB 5-11 239 23 R North Alabama
16 Blake, Emory WR 6-0 192 22 1 Auburn
60 Bond, Travis OG 6-6 329 23 1 North Carolina
86 Brodine, Mason TE 6-7 284 26 1 Nebraska-Kearney
2 Cowan, Bobby P 6-4 219 24 1 Idaho
84 Cunningham, Justice TE 6-3 258 23 1 South Carolina
64 Hooey, Sean T 6-9 304 23 1 Cincinnati
49 Hus, Jorgen LS 6-1 232 24 1 Regina
56 McSurdy, Caleb LB 6-1 237 24 1 Montana
87 Moe, T.J. WR 5-11 204 23 1 Missouri
59 Sabino, Etienne OLB 6-2 247 23 1 Ohio State
53 Steward, Phillip LB 6-1 235 23 1 Houston
19 Veltung, Justin WR 5-11 182 23 1 Idaho
35 Woodard, Darren CB 5-11 175 24 1 UTEP
From that list; TJ Moe
I will also say C Bryant seems to have a real chip on his shoulder. Those kind of guys have a chance to stand out!