Ancient Greece AR 16.72 G 25.8mm For ID

But almost certainly a fake
The flan crack is very odd
On a bronze coin it might be an indication of it being genuine
But on a silver tetradrachm ?
The edge at 2 o'clock appears flat
Which be indicating a casting
But the crack seems to point to a "struck" coin
Either way there are some serious issues !

BTW
If it were genuine
It would be worth more than all the other coins you probably own !
Which in itself is reason to doubt

For comparison to a specimen condemned in the IBSCC Bulletin on Counterfeits BOCS Vol 222, #2, 1997.

Allowing for differences in the perspective/photography, there are lots of correspondences in details - with the exception of the two dolphins lower left and some of the lettering.

Whether this suggests that both the IBSCC and OP versions are fake, and demonstrate some reworking of the mold/dies during the run - or whether it suggests that the OP is legit, and that the forgers used a coin with the same OP dies as the prototype, but then reworked two of the dolphins for some reason, I dunno.

Curiouser and curiouser ......
I see things in that edge that definitely look suspect
I also see things in the crack covering the body of the horse that look like it lacks depth
At the same time I see the section nearer the edge looking like a genuine flan crack
In fact it has every appearance of being a fouree
Which could go a long way explaining why such a fracture would appear on a silver tetradrachm

So there are reasons to disbelieve as well believe
I think you should seriously consider sending it out for authentication
If the coin is genuine you will need more than just a promise to get full value
I assume you have an idea what these "beasties" fetch on the open market !
In fact
If it is a fouree of a genuine tetradrachm it may well fetch more than you might expect