  The Event Horizon Telescope (EHT) collaboration recently unveiled the first
image of the supermassive black hole M87*, which exhibited a ring of angular
diameter $\theta_{d}=42 \pm 3 \mu as$, a circularity deviation $\Delta C \leq
0.1$, and also inferred a black hole mass of $M=(6.5 \pm 0.7) \times 10^9
M_\odot $. This provides a new window onto tests of theories of gravity in the
strong-field regime, including probes of violations of the no-hair theorem. It
is widely believed that the Kerr metric describes the astrophysical black
holes, as encapsulated in the critical but untested no-hair theorem. Modeling
Horndeski gravity black holes -- with additional hair parameter $h$ besides the
mass $M$ and spin $a$ of the Kerr black hole -- as the supermassive black hole
M87*, we observe that to be a viable astrophysical black hole candidate, the
EHT result constrains ($a$, $h$) parameter space. However, a systematic bias
analysis indicates rotating Horndeski black hole shadows may or may not capture
Kerr black hole shadows, depending on the parameter values; the latter is the
case over a substantial part of the constrained parameter space, allowing
Horndeski gravity and general relativity to be distinguishable in the said
space, and opening up the possibility of potential modifications to the Kerr
metric.
