Which ping implementation is Cygwin using?

It's Windows ping. Just run:

which ping 

You'll get:

/cygdrive/c/WINDOWS/system32/ping

Cygwin ping is available on ping package from inetutils source

$ which ping
/usr/bin/ping 

$ cygcheck -f `which ping`
ping-1.9.4-1

x86_64 $ ping --help
Usage: ping [OPTION...] HOST ...
Send ICMP ECHO_REQUEST packets to network hosts.

 Options controlling ICMP request types:
      --address              send ICMP_ADDRESS packets (root only)
      --echo                 send ICMP_ECHO packets (default)
      --mask                 same as --address
      --timestamp            send ICMP_TIMESTAMP packets
  -t, --type=TYPE            send TYPE packets

 Options valid for all request types:

  -c, --count=NUMBER         stop after sending NUMBER packets
....