Puppy refusing walks

Hi everyone,

Have a wonderful greyhound pup which is 4 months old. We got him 12 days ago.

First days he was happy to go on walks and we were really focused on potty training.

Now 2 days ago he started getting deathly afraid of walks. He doesn’t want to leave the house if his leash is on and he doesn’t want to leave the apartment building. He starts crying and wants to go back inside.

We are giving him some high value treats when outside and when he walks alongside us but I’m worried this is something that will stick if we can’t get past this quickly.

Any advice?