I need help on making my horses fatter

its not really my horse its my parents horse. She is skinny & will NOT gain weight, i had this mare for 4 years before my parents bought her from me, she was skinny then, my cousins had her before me (about 3 years) she was skinny then. we i bought her i tried senior feed, weight gainner, & veg. oil, none have put 1 pound on her!!! the vet looked at her & said she might need her teeth flouted, we got that done she didnt gain weight from that, my parents worm her & so have i when i had her. someone PLEASE help us she looks like we are starving her when were not!!! we dont ride her so she is just out in the pasture, so we dont no whats going no with her. PLEASE HELP

Have the vet draw out a worming routine for your area - and stick to it

Have her teeth checked every 6 months

Feed her supplemented hay - alfalfa preferred for skinnier horses

Give her 5 lbs of beet-pulp daily

Just because she's in pasture doesn't mean she's eating right and what she is eating she's wasting in walking around. If you REALLY want to put the weight on, take her out of pasture and put her in a paddock or corral (no larger then say 100 x 100.) If she's with other horses, maybe they push her around making her exercise and expend calories. Separate her from the others at least by a fence.

Could be that's just how she is and you need to keep her filled with good quality calories (alfalfa, beet-pulp, weight gain, etc)