You wouldn't want quotes around the "false". I think it might be something like:
<?php
if ($row_WADAtbl_posts['video']) {
if (strpos($row_WADAtbl_posts['video'],"iframe") === false) {
?>
<div class="video-container">
<?php echo($row_WADAhorses['horse_video']); ?>
</div>
<?php
} else {
?>
<?php echo($row_WADAhorses['horse_video']); ?>
<?php
}
}
?>